Introduction: Why Most Beginners Lose Fish in the First Month
Every week, hundreds of fish are purchased from markets in Phnom Penh, Siem Reap, and across Cambodia — and a large number of them die within days. Not because they were unhealthy, and not because the new owner didn't care. They die because of a set of ten completely predictable, completely preventable mistakes that almost every beginner makes. This guide exists so that you don't make them.
Keeping tropical fish in Cambodia comes with its own specific challenges. The heat alone — temperatures regularly hitting 32 to 35 degrees Celsius from March through June — creates water chemistry conditions that are unforgiving of beginner errors. Add in Phnom Penh tap water that is treated with chlorine and chloramine, and the gap between success and failure becomes very narrow if you don't know what you're doing.
The good news is that every single mistake on this list is avoidable with basic knowledge and a small investment in the right supplies. Whether you are setting up your first ten-litre bowl or a sixty-litre community tank, these ten lessons will save you money, save your fish, and turn what is often a frustrating first experience into one that is genuinely rewarding.
Read this guide before you buy your first fish. Bookmark it and return to it when something goes wrong. And if you're already keeping fish and things aren't going well — start from mistake number one, because the answer is almost certainly somewhere on this list.
- ✦Read this entire guide before purchasing any fish — setup errors cannot be fixed once fish are inside the tank.
- ✦Keep a simple notebook of water changes, feeding amounts, and any fish deaths — patterns will tell you which mistake you're making.
- ✦Budget for supplies first: dechlorinator, a test kit, and a proper filter matter more than decorations.
Mistakes 1 and 2: The Tank Was Never Ready for Fish
Mistake number one is starting without cycling the tank — and it is the single most deadly error a beginner can make. Cycling refers to establishing a colony of beneficial bacteria in your filter that convert toxic ammonia (produced by fish waste and uneaten food) into nitrite, and then into the much safer nitrate. In a brand-new tank, those bacteria do not exist yet. When you add fish to an uncycled tank, ammonia builds up within 24 to 48 hours to levels that burn the fish's gills, damage their organs, and kill them — sometimes within three to five days. The fish look fine on day one and are dead by day four, and most beginners have no idea why.
The correct approach is to run the tank empty for two to four weeks before adding any fish. Add a source of ammonia — a small pinch of fish food each day, or a few drops of pure ammonia — and let the bacterial colony establish itself. You will know the cycle is complete when you test the water and find zero ammonia, zero nitrite, and a measurable level of nitrate. Only then is it safe to add fish. In Cambodia's hot climate, cycling actually happens faster than in cooler countries — beneficial bacteria thrive above 28 degrees Celsius — so a three-week cycle is very achievable if you start correctly.
Mistake number two sounds logical but is wrong: buying a small tank because it seems easier to manage. A one-litre bowl or a five-litre cube is actually far more dangerous for fish than a forty-litre tank. Water volume is the buffer that protects fish from rapid chemical changes. In a small tank, one dead fish or one day of overfeeding can spike ammonia to lethal levels within hours. In a larger tank, the same event is diluted across much more water and gives you time to react. The rule of thumb used by experienced fishkeepers is this: the larger the volume, the more stable the chemistry, the healthier the fish.
In Cambodia's climate, small tanks also heat up dramatically fast. A five-litre bowl sitting on a table near a window in Phnom Penh can reach 36 degrees Celsius within an hour of morning sun — a temperature that will kill most tropical fish within a day. A forty-litre or larger tank with a lid takes much longer to change temperature, giving your fish a far more stable environment. Start with at least twenty litres. Sixty litres is ideal for beginners keeping community fish.
- ✦Use a cycling starter product (available at fish shops for around 3,000-5,000 KHR) to speed up the bacterial colonisation process.
- ✦Never add more than 25% of your planned fish load on day one — the bacteria need time to grow proportionally to the waste load.
- ✦In Cambodia's hot season, monitor your small tank temperature daily — even a clip-on fan over the water surface helps reduce temperature by 1-2 degrees.
Mistakes 3 and 4: Food and Water Chemistry Errors
Mistake number three is overfeeding, and it is the most common ongoing mistake in established tanks. Fish have stomachs roughly the size of their eye. They do not need much food, and they will eat as much as you give them because they are instinctively programmed to eat whenever food is available. Uneaten food sinks to the gravel, begins decomposing within hours, and produces ammonia. In Cambodia's warm water — where bacterial decomposition happens faster than in temperate climates — a tank can show a dangerous ammonia spike within twelve hours of a single overfeeding event.
The two-minute rule is the most reliable guideline: feed only what your fish can consume completely within two minutes, and feed once or twice per day at most. If food is still floating or sinking after two minutes, you fed too much. Remove the excess immediately using a small net or a gravel siphon. Many Cambodian fishkeepers make the mistake of feeding more because the fish look hungry and gather at the glass — this is normal fish behaviour and does not mean they are starving. Resist it.
Mistake number four is not dechlorinating tap water before adding it to the tank. Phnom Penh's municipal water supply is treated with chlorine and, increasingly, chloramine — both of which are toxic to fish and to the beneficial bacteria in your filter. A water change done with untreated tap water can set your cycle back by days and stress your fish significantly. Stressed fish develop weakened immune systems and become vulnerable to ich, fin rot, and bacterial infections within a week.
Dechlorinator is inexpensive — a small bottle costs roughly 4,000 to 8,000 KHR at most fish shops and treats hundreds of litres of water. Add the correct dose to your bucket of tap water before pouring it into the tank, every single time. Do not skip this step even once. If you run out of dechlorinator, let tap water sit in an open bucket for 24 hours to off-gas chlorine naturally — but note that this does not remove chloramine, which is why having dechlorinator on hand at all times is essential in Phnom Penh.
- ✦Set a phone alarm to remind yourself to feed fish — routine feeding at fixed times prevents the temptation to add extra food throughout the day.
- ✦Keep a spare bottle of dechlorinator at home at all times — never do a water change without it.
- ✦If you notice your fish gasping at the surface after a water change, untreated tap water is almost always the cause — do a 30% water change immediately with properly treated water.
Mistakes 5 and 6: Stocking and Filter Maintenance Errors
Mistake number five is adding too many fish at once. When you introduce a large group of fish into a tank simultaneously, two dangerous events happen in quick succession. First, the ammonia load immediately overwhelms your bacterial colony, which cannot multiply fast enough to process the waste from fifteen fish when it was only maintaining three. Second, the oxygen in the water can drop sharply during what is called a bacterial bloom — a rapid multiplication of heterotrophic bacteria that consumes dissolved oxygen faster than your air pump can replace it. In Cambodia's warm water, dissolved oxygen is already naturally lower than in cooler climates, making this risk even more significant.
The correct approach is the 25% rule: add no more than one quarter of your planned fish population at a time, then wait at least two weeks before adding the next group. This gives your beneficial bacteria time to grow proportionally to the new waste load. It also gives you time to observe the fish you just added for signs of disease before they have had a chance to infect others. If you are stocking a forty-litre tank with twenty small fish, add five, wait two weeks, add five more, and continue the pattern.
Mistake number six is one that experienced fishkeepers cringe at: cleaning the filter under the tap. It seems logical — the filter is dirty, so rinse it under clean running water. But that running tap water contains chlorine, and it will kill every single beneficial bacterium living in your filter media within minutes. Your cycle is effectively destroyed, and your tank becomes dangerously unsafe for fish within hours of this seemingly helpful action.
The correct method is to remove the filter media and rinse it gently in a bucket of old tank water — water you have just removed during your regular water change. Squeeze the sponge a few times, swirl the ceramic rings, and that is enough. You are not trying to make the filter media look clean — you are trying to remove solid waste while preserving the bacterial biofilm that makes your filter work. Never use soap, boiling water, or tap water on any filter media, ever.
- ✦Keep a written stocking plan before you buy any fish — know your target population and your 25% addition schedule before you visit the market.
- ✦Label your water change bucket with permanent marker so it is never accidentally used for anything else — soap residue in a bucket will kill fish.
- ✦Never clean filter media and do a large water change on the same day — these two actions together create the largest possible stress event for your tank's bacterial colony.
Mistake 7: Skipping Quarantine — The Mistake That Wipes Entire Tanks
Mistake number seven is not quarantining new fish, and it is arguably the most expensive mistake on this list in terms of what it costs you. Fish sold at Cambodian markets — particularly the open-air fish stalls common in Phnom Penh and larger provincial towns — are often housed in crowded conditions with shared filtration and no separation between sick and healthy animals. Ich (white spot disease), velvet, and bacterial infections are common in market fish and may not show visible symptoms for three to five days after purchase, while the fish is already contagious.
When you add an infected fish directly to your display tank, you expose every other fish to the pathogen immediately. Ich in particular spreads at an exponential rate in warm water — Cambodia's 30-plus degree water temperatures are ideal for the ich parasite lifecycle, which can complete a full generation in as little as 24 hours at 32 degrees. A tank that had fifteen healthy fish can be completely lost within five to seven days of introducing one infected fish. The financial loss can reach 50,000 to 150,000 KHR or more, depending on what species you were keeping.
The solution is a quarantine tank: a separate, simple setup of ten to twenty litres with a small sponge filter, a heater if needed, and no decorations that are hard to disinfect. All new fish spend a minimum of two weeks in the quarantine tank before entering the display tank. Observe them daily for spots, clamped fins, flashing behaviour, or abnormal swimming. Treat any symptoms immediately in quarantine, where medication costs far less and where your display tank's beneficial bacteria are not exposed to the chemicals.
A basic quarantine setup costs less than 80,000 KHR to assemble and will pay for itself the first time it intercepts a disease outbreak. Think of it as insurance — you may go months without needing it, but the one time you do, it saves everything. This is especially important when buying fish from new suppliers or from market stalls rather than dedicated aquarium shops where fish health standards are typically more carefully maintained.
- ✦Keep a simple quarantine log: date purchased, source, species, and any symptoms observed during the two-week hold.
- ✦Medicate only if you observe symptoms — do not treat prophylactically unless advised by a specialist, as some medications damage your quarantine tank's bacteria.
- ✦After the quarantine period, acclimate fish to your display tank slowly over 30-45 minutes using the drip method or by floating the bag — temperature and pH differences between tanks stress fish during transfer.
Mistakes 8 and 9: Compatibility and Placement Errors
Mistake number eight is choosing fish based on size rather than temperament. Many beginners assume that a small fish cannot harm a large fish, but this reasoning is badly wrong. A single male betta will shred the fins of any fish it perceives as a rival, regardless of that fish's size. Tiger barbs, which grow to only about seven centimetres, will relentlessly nip the fins of large, slow angelfish until the angelfish develop fin rot and die from secondary infection. Researching temperament before purchasing fish is not optional — it is the difference between a peaceful community tank and a tank full of stressed, injured animals.
In Cambodia, where the popular fish at most markets are a combination of local species and imported varieties, incompatibility issues are common because sellers do not always provide accurate information. The safest approach for beginners is to choose one of the well-documented beginner community combinations — for example: guppies with tetras and corydoras, or a single betta with peaceful bottom dwellers like kuhli loaches. Avoid cichlids, oscars, and flowerhorns in community tanks entirely until you have at least a year of experience and a dedicated species tank.
Mistake number nine is placing the aquarium in direct sunlight. A tank positioned near a window in Phnom Penh during the hot season receives intense solar radiation that causes two simultaneous problems: algae blooms and dangerous temperature spikes. Algae can turn a tank green and coat every surface within two to three days of direct sun exposure. More seriously, water temperature in a sun-exposed tank can rise from a safe 28 degrees to a lethal 36 or 37 degrees within an hour in March or April — killing fish faster than almost any other single event.
Place your tank in a location with bright ambient light but no direct sun rays touching the glass at any time of day. If your only available space receives afternoon sun, use a blackout curtain or tank backing on that side of the glass. In Cambodia's climate, the goal is actually often to keep the tank cool rather than warm — running a small fan across the water surface can reduce temperature by one to two degrees through evaporation, which is a significant and inexpensive intervention during the hot season.
- ✦Before buying any new fish, search its name plus 'tank mates' online and confirm it is compatible with every species already in your tank.
- ✦Use a thermometer on your tank every morning during Cambodia's hot season — a temperature above 32 degrees Celsius needs immediate action.
- ✦A simple clip-on desk fan positioned to blow across the water surface costs around 15,000-25,000 KHR and can reduce tank temperature by 1-2 degrees — a worthwhile investment from March to June.
Mistake 10: Not Testing the Water — The Root of All Unsolved Problems
Mistake number ten is not testing water parameters, and it ties every other mistake on this list together. Ammonia, nitrite, nitrate, and pH are invisible. You cannot see them, smell them in most cases, or feel them. The only way to know what is happening in your tank's chemistry is to measure it. Beginners who skip water testing are essentially flying blind — they notice that fish are dying but have no idea whether the cause is ammonia poisoning, pH crash, or nitrite toxicity. Without a test kit, every problem looks the same and every treatment attempt is a guess.
A basic liquid test kit — covering ammonia, nitrite, nitrate, and pH — costs between 80,000 and 150,000 KHR at specialist aquarium shops in Phnom Penh. This is a one-time purchase that lasts for years and provides the data you need to make informed decisions. Test your water weekly during the first three months, and any time fish behave abnormally — gasping, hiding, loss of colour, or erratic swimming. Knowing that your ammonia is at 0.5 ppm tells you exactly what action to take: a water change, reduced feeding, and filter assessment. Guessing without data leads to wrong treatments that make problems worse.
In Cambodia, water quality issues from tap water add an additional layer of complexity. Phnom Penh's tap water pH can vary, and during rainy season the municipal supply sometimes changes its treatment chemistry. A pH crash — when the buffering capacity of your water is exhausted and pH drops rapidly — can kill a tank of fish overnight without any visible warning signs beforehand. Regular testing is the only early warning system available to you. Consider it the most important tool in your fishkeeping kit, more important even than any decoration or expensive equipment.
The habit of testing water and recording results in a simple notebook transforms fishkeeping from guesswork into a skill. Over time, you will see patterns — pH drops after a week without a water change, ammonia rises if you skip a feeding routine, nitrate climbs faster in a heavily planted tank. This data makes you a better fishkeeper with every test you run, and it dramatically reduces the chance of losing fish to preventable water chemistry events.
- ✦Test water the same day each week and write results in a notebook — three months of data will show you seasonal patterns specific to your local tap water.
- ✦If you cannot afford a full liquid test kit immediately, buy ammonia test strips as a minimum — ammonia is the most critical parameter to monitor in the first three months.
- ✦Share your test results on Cambodia aquarium forums — experienced local keepers can spot problems in your numbers that you might miss.
Final Words: Start Right and Your Fish Will Thrive — 4848 One Shop Can Help
These ten mistakes are responsible for the overwhelming majority of fish deaths in beginner tanks across Cambodia. Every one of them is avoidable with basic knowledge and a modest investment in the right supplies. The most important lesson from this list is that fishkeeping success is almost entirely determined before the first fish enters the tank — in how you cycle the water, how you choose your tank size, where you place it, and what test kit you have on hand. Get these fundamentals right and you will rarely need to troubleshoot anything more serious.
Cambodia's tropical climate actually offers one genuine advantage to aquarium keepers: the ambient temperature keeps your tank warm without a heater for most of the year, which reduces equipment costs and energy bills. The challenge is managing the upper end of that temperature range during the hot months — but as this guide has shown, that challenge has straightforward, affordable solutions. The fundamentals of fishkeeping are the same everywhere in the world; applying them to Cambodia's specific conditions is simply a matter of awareness and preparation.
If you are ready to start your aquarium journey with confidence, or if you want to rebuild a tank that has not been going well, we invite you to visit 4848 One Shop — Cambodia's dedicated tropical fish community. Our team can advise you on which fish species are best suited to Cambodia's climate, which dechlorinator products work best with Phnom Penh tap water, and how to build a beginner setup that gives your fish the best possible chance. We also stock quarantine supplies, water test kits, and cycling products so you can start every tank the right way.
Keeping fish well is a skill that compounds over time. Every tank you set up correctly teaches you something for the next one. Every water test you run builds your understanding of chemistry. Every fish that thrives in your care is proof that the knowledge you invested in this guide was worth it. Start right, be patient, and the hobby will reward you for years to come.