Why Cambodia's Rainy Season Is a Different Beast Entirely
Every year from May through October, Cambodia's rainy season transforms the environment in ways that affect your aquarium far beyond simply adding more humidity to the air. International fish-keeping guides — the ones written for hobbyists in Europe, Japan, or North America — simply do not account for what Cambodian keepers face during this period. The combination of extreme heat, sudden storms, unreliable power, and changing municipal water quality creates a unique set of pressures on your tank and your fish.
In Phnom Penh and across the country, fish keepers in ground-floor apartments and shophouse units face the very real risk of flooding during heavy downpours. Street drains overflow, water seeps under doors, and electrical equipment sitting at floor level becomes a serious hazard. This is not something a guide written for a basement fish room in Germany will ever mention, yet it is one of the first things every Cambodian keeper must plan for each May.
Beyond flooding, the season introduces a cascade of interacting problems: humidity climbs into the 85–95% range, afternoon thunderstorms cut power for anywhere from ten minutes to several hours, and the temperature inside a Cambodian home can swing by 3 to 5 degrees Celsius between a hot noon and a cool rainy evening. Each of these stressors alone is manageable. All of them together, hitting your tank simultaneously, is where fish are lost.
This guide is written from the perspective of keepers who live in this climate and keep fish through it every year. We will walk through each major rainy-season challenge in practical terms — what causes it, how it affects your fish, and what you can do right now, before the next storm arrives.
- ✦Mark your calendar for May 1 as 'rainy season prep day' — inspect all electrical placements and test your emergency equipment before the season begins.
- ✦Keep a local weather app with storm alerts active on your phone — a 30-minute warning before a heavy storm is enough time to switch on a battery air pump and close partially open lids.
- ✦Join a local Cambodian fish-keeping Facebook group or Telegram channel — local keepers share real-time storm alerts and flooding warnings faster than any official source.
Flooding Risk and Electrical Safety for Floor-Level Equipment
One of the most underappreciated dangers during Cambodia's rainy season is the position of your electrical equipment relative to the floor. Power strips, filter plugs, heater cords, and lighting transformers that sit directly on the floor in a ground-floor unit or a fish room with poor drainage can become life-threatening hazards when even two or three centimeters of water enters the space. This is not an extreme scenario — it happens regularly in parts of Phnom Penh, Siem Reap, and Kampot during peak rainy-season storms.
The first step every Cambodian keeper should take before the season begins is to raise all electrical connections off the floor. A simple wooden platform, a plastic shelf unit, or even zip-tied cord management that routes cables up the wall and connects at a raised power strip can eliminate this risk entirely. Aim to keep every plug point and power strip at least 30 centimeters above your floor level. In lower-lying areas of Phnom Penh, 50 centimeters is a safer target.
Ground Fault Circuit Interrupter outlets — known as GFCI outlets — are the international standard for any room where water and electricity coexist. In Cambodia, these are available at hardware stores around Phsar Orussey and through electrical supply shops on Street 182. They are not expensive, typically ranging from around $8 to $15 USD (roughly 32,000 to 60,000 KHR), and they can prevent electrocution if water does contact a live outlet. If you have any electrical connections near your tank that are not GFCI-protected, installing them before rainy season is one of the best investments you can make.
Finally, plan a mental 'flooding checklist' that you can run quickly when a storm warning comes. Know which plugs you will pull first, where your battery backup equipment is stored, and whether your tank can safely run for two to three hours on an air pump alone if you need to shut down all mains-connected equipment. Having this plan in your head costs nothing and could save both your fish and your life.
- ✦Route all power strips up the wall using cable clips — aim for a minimum of 30 cm above floor level, 50 cm in flood-prone areas.
- ✦Install GFCI outlets for all aquarium electrical connections — available at Phsar Orussey hardware stalls for around $8–15 USD.
- ✦Keep a laminated 'storm checklist' near your fish room listing the order in which to disconnect equipment if flooding begins.
Humidity, Tank Covers, and the Evaporation Paradox
Cambodia's rainy season pushes indoor humidity into ranges that European and American fish-keeping guides simply never model. When ambient humidity sits between 85% and 95% for weeks at a time, the relationship between your tank lid and your water quality changes in ways that are not always intuitive. The usual advice — 'partially open your lid for gas exchange' — needs to be applied with more nuance during this season.
The paradox is this: leaving a lid partially open during high-humidity periods does still allow evaporation and water loss, even when the air feels saturated. Warm tank water at 30°C or above will continue to evaporate into even very humid air, meaning your water level can drop faster than you expect during a warm afternoon. Top-up water should be prepared and dechlorinated in advance rather than relying on a last-minute pour of tap water.
On the other hand, fully sealing a tank during the rainy season to minimize evaporation creates its own risk: reduced gas exchange. Oxygen levels drop in a sealed tank, especially one with a heavy bioload or dense planting, and CO2 from fish respiration accumulates. In Cambodian conditions — where the ambient temperature is already keeping fish metabolisms running high — this can cause visible stress signs such as fish gasping at the surface within just a few hours of a complete seal.
The practical balance for most Cambodian keepers is a lid that is approximately 70 to 80 percent covered, with a reliable method of checking and topping up water every two to three days. A dedicated top-off container of pre-treated water sitting beside the tank makes this routine fast and safe. During periods of extended power outage when your filter and surface agitation are off, lean toward a slightly more open lid to compensate for the lost surface movement.
- ✦Pre-fill a 5–10 liter container with dechlorinated tap water beside each tank so top-offs take under a minute.
- ✦Use a floating thermometer with a visible water-level marker line — when the line drops, it is time to top off.
- ✦Avoid fully sealing tanks during power outages — reduced gas exchange in a sealed tank without a running filter stresses fish quickly.
Power Outages During Storms: Keeping Fish Alive When the Lights Go Out
Every Cambodian fish keeper who has kept the hobby through a few rainy seasons has experienced the specific dread of watching their tank go still and dark during a heavy evening thunderstorm. Power cuts during storms in Cambodia can last anywhere from a few minutes to several hours, and in rural provinces, overnight outages are not uncommon. Understanding the priority order for fish survival when power goes out is the difference between waking up to a healthy tank and waking up to dead fish.
The survival priority is oxygen first, temperature second. This is a rule that international guides do state correctly, but it is worth repeating for the Cambodian context. In a tank without a running filter or air pump, oxygen depletion begins within minutes in a warm, heavily stocked setup. A battery-powered air pump running a simple sponge filter is the single most important piece of emergency equipment a Cambodian fish keeper can own. These are available locally at fish markets in Phnom Penh for around $5 to $12 USD (20,000 to 48,000 KHR) and should be tested regularly, not just purchased and stored.
Temperature, the second concern, is more forgiving than most beginners expect during short outages. Cambodia's ambient temperature during the rainy season rarely drops below 26–27°C even on a cool rainy night, which means most tropical fish will tolerate a few hours without a heater far better than they will tolerate a few hours without oxygen. Where temperature becomes critical is in households running strong air conditioning during the day — a tank that has been sitting at 28°C with AC running can drop toward 24°C in a cooled room during a long overnight outage, which creates stress for sensitive species.
For keepers with larger investments in sensitive fish — discus, high-end bettas, or show-grade cichlids — a small UPS (uninterruptible power supply) unit capable of running a compact filter and heater for two to four hours is a worthwhile rainy-season investment. These are available through electrical supply shops in Phnom Penh and online stores that deliver within Cambodia for $30 to $80 USD depending on capacity.
- ✦Own at least one battery-powered air pump per tank and test it monthly — dead batteries during a storm are as bad as no pump at all.
- ✦Label your battery air pump with the date batteries were installed — replace them at the start of each rainy season regardless.
- ✦If you run AC during the day, insulate your tank with a blanket or foam panels during a long outage to slow temperature drop.
Temperature Swings: The Hidden Danger in Air-Conditioned Cambodian Homes
Cambodia's rainy season temperature profile is more complex than most beginner guides acknowledge. The challenge is not just heat — it is the gap between the heat and what replaces it. In many Phnom Penh households and apartments, air conditioning runs during the hottest part of the day, keeping indoor temperatures around 24 to 26°C. Then a rainy evening cools the air naturally, AC gets switched off, and the room may settle at 27 to 29°C overnight. For the aquarium, this daily cycle creates a yo-yo effect that plays out across the water column.
A swing of 3 to 5°C within a single day is within the physiological stress threshold for many popular tropical fish. Discus are famously sensitive, preferring stable temperatures within a 1°C range. Bettas are hardier but exhibit visible stress and reduced immune function when temperatures oscillate daily. Even community fish like tetras and rasboras, which tolerate a wider range, will show increased disease susceptibility after weeks of daily temperature cycling.
The most practical solution for Cambodian keepers running air conditioning is to set the aquarium heater to a stable target temperature that sits at or slightly above the minimum the AC brings the room to. If your AC holds the room at 25°C during the day, set your heater to 27 to 28°C. The heater will not run during the warm part of the day and will kick in when temperatures drop, smoothing out the swing to less than 1°C. This costs almost nothing in electricity but makes a significant difference to fish health.
For keepers in homes without air conditioning, the rainy season temperature profile is generally less problematic. Ambient temperatures in a non-AC home in Phnom Penh during the rainy season typically stay between 27 and 32°C, which is a perfectly acceptable range for most tropical fish without any heater involvement. The concern in non-AC homes is the upper end — if your tank sits in direct sunlight or in a room that heats above 34°C, adding a small fan blowing across the water surface can bring temperatures down by 2 to 3°C through evaporative cooling.
- ✦Set your aquarium heater to a stable minimum rather than turning it off during AC periods — a stable 28°C is better than swings between 25°C and 30°C.
- ✦Place a maximum-minimum thermometer in the tank for one week during early rainy season to understand your actual daily swing before adjusting equipment.
- ✦For tanks near windows, use reflective window film or a shade cloth to block direct sun and reduce peak temperature spikes during hot afternoons.
Phnom Penh Tap Water During the Rainy Season: Test More, Assume Less
Phnom Penh's tap water treatment system handles one of the most challenging raw water sources in Southeast Asia, and during the rainy season it faces its hardest test of the year. Heavy rainfall causes increased turbidity in source water from the Tonle Sap and Mekong river systems, and the municipal treatment plant must adjust its processes accordingly. For fish keepers, this means that the tap water coming out of your pipe in June is not the same chemistry it was in February.
The most notable change Cambodian keepers observe during the rainy season is inconsistent chlorination levels. In some periods, chlorine may be higher than usual as the plant responds to higher turbidity; in other periods, particularly during very heavy continuous rain events that strain the system, chlorination consistency can become unpredictable. The practical advice is straightforward but important: test your tap water with a chlorine test kit before every water change during rainy season, rather than assuming your standard dechlorinator dose is sufficient.
Beyond chlorination, pH and hardness can also shift during heavy rain periods as source water chemistry changes. If you keep fish with specific pH requirements — South American species preferring soft acidic water, or African cichlids requiring hard alkaline conditions — more frequent parameter testing during the rainy season is wise. A basic API or similar test kit covering pH, ammonia, nitrite, nitrate, and chlorine will give you the data you need to make safe water change decisions.
One practical habit that experienced Cambodian keepers develop is preparing change water the evening before rather than immediately before a water change. This allows any chlorine to off-gas partially, gives you time to add dechlorinator and let it work, and lets you check temperature matching before the water goes into the tank. During rainy season, this preparation step matters more than at any other time of year.
- ✦Keep a dedicated chlorine test kit beside your tank and test every tap water batch during rainy season — do not assume your standard dose is sufficient.
- ✦Prepare change water 12–24 hours in advance during peak rainy season months (June–September) for extra safety.
- ✦If you notice your fish looking stressed after a water change, test the new water for pH and chlorine before the next change rather than guessing.
Mosquito Prevention in Your Fish Room: An Overlooked Rainy Season Problem
Cambodia's rainy season creates ideal mosquito breeding conditions, and a fish room — with its collection of open containers, buckets of aging water, and partially covered tanks — can become an unintended breeding ground. Beyond the obvious personal nuisance, mosquito larvae in your fish tank are not actually a problem (most fish eat them enthusiastically), but adult mosquitoes breeding in your fish room are a health concern for your household and a sign of standing water management that needs attention.
The primary targets for mosquito control in a fish room are not the main display tanks but the secondary water sources: buckets holding aging change water, lids left upside-down that collect rainwater, empty containers, and any equipment sitting outside the tank that holds standing water. Making a weekly sweep of your fish area to dump, cover, or treat any standing water source takes less than five minutes and significantly reduces the breeding population.
For open tanks or vats containing fish — such as breeding setups or holding containers used by small-scale sellers — tight-fitting window screen mesh across the top is the best solution. It allows air exchange and humidity regulation while preventing adult mosquitoes from accessing the water surface to lay eggs. This mesh is inexpensive and widely available at building supply shops across Cambodia for a few thousand KHR per meter.
It is worth noting that chemical mosquito treatments such as coils, plug-in repellents, and sprays should be used with extreme caution around aquariums. Many of these products contain pyrethroids or other compounds that are highly toxic to fish even at very low concentrations in the air. If you use mosquito coils or sprays in a room with open tanks, ensure strong ventilation or cover all open tank surfaces before application and for at least an hour afterward.
- ✦Do a weekly standing water audit of your fish room — dump any water sitting in non-essential containers.
- ✦Cover open breeding vats and holding tanks with window screen mesh — it costs almost nothing and blocks mosquito access completely.
- ✦Never use mosquito coils or spray in an enclosed fish room with open tanks — pyrethroids are lethal to fish at concentrations safe for humans.
Local Fish Market Quality and Where to Find Good Stock During Rainy Season
Rainy season is not the ideal time to purchase fish from Cambodia's informal local fish markets, and experienced keepers here know this from hard experience. The combination of transport stress in heat, holding tanks that may not have power backup during storms, and the general increase in disease pressure that all fish experience during unstable weather creates a higher-than-normal rate of sick or recently stressed fish at market stalls. This does not mean you cannot buy fish during the rainy season — it means you must inspect more carefully and quarantine without exception.
When visiting local markets, look specifically for signs of rainy-season stress: fish sitting unusually still at the bottom, clamped fins, visible white spots or fraying, cloudy eyes, or labored breathing. Sellers at reputable shops will not be offended if you watch a tank for a few minutes before purchasing — any fish keeper who knows their stock will welcome that attention. For common species available locally, prices in Phnom Penh range from around 2,000 to 5,000 KHR for basic community fish up to $5 to $30 USD or more for quality bettas and imported cichlids.
Quarantine during the rainy season should be longer than at other times of year — aim for three weeks minimum rather than the two weeks many guides suggest. The combination of transport stress, weather stress, and the generally elevated disease pressure of the season means that latent infections take longer to manifest. A quarantine tank running a basic sponge filter, a stable temperature, and no other fish gives new arrivals the best chance to recover and reveal any issues before they enter your main display.
For the highest quality and best-sourced tropical fish in Cambodia, buying from a specialist shop rather than a general market stall makes a significant difference during the rainy season. 4848 One Shop maintains healthy, well-conditioned stock year-round and can advise you on species that are best suited to riding out the rainy season challenges in your specific setup. Whether you are looking to add to an existing tank or start fresh, visiting 4848oneshop.zakgt.net or coming in person is the most reliable way to find fish that are genuinely ready for a Cambodian rainy-season home.