Cambodia's Dry Season: What Actually Happens to Your Tank
Cambodia's dry season runs from roughly November through April, with the peak heat period falling between March and mid-April before the first rains arrive. During this window, ambient temperatures in Phnom Penh homes and apartments regularly reach 34 to 37 degrees Celsius in the afternoon, with outdoor temperatures exceeding 40°C on the hottest days of April. Without active cooling, aquarium water temperature tracks ambient temperature within 1 to 2 degrees — meaning that a tank sitting in a typical Cambodian living room can reach 34 to 36°C during afternoon peak hours.
Most tropical fish are comfortable between 24 and 29°C. Some hardy species can tolerate short exposure to 32 or 33°C. At 35°C and above, nearly all common aquarium fish begin to show thermal stress — reduced appetite, erratic swimming, surface gasping as dissolved oxygen drops, and immune system suppression that makes them vulnerable to opportunistic bacterial infections. A tank that spends several consecutive weeks running hot during Cambodia's dry season will show the cumulative cost in reduced fish vitality, shorter lifespans, and increased disease susceptibility.
The second major dry-season effect is evaporation. In Phnom Penh's dry season air — low humidity, often below 50% — an uncovered 200-liter aquarium can lose 2 to 4 liters of water per day through evaporation. Over a week, this represents a 10 to 15 percent volume reduction, but more importantly, everything that was dissolved in that evaporated water — minerals, salts, hardness compounds — remains in the tank and becomes more concentrated. pH rises, carbonate hardness (KH) rises, and the tank's chemistry shifts away from the conditions your fish were acclimated to.
Understanding these two mechanisms — heat and evaporation — as connected rather than separate challenges is the key insight for dry-season management. A keeper who only tops off evaporated water without addressing temperature is still managing a hot tank. A keeper who cools the tank without managing evaporation is watching their chemistry drift. Effective dry-season management addresses both simultaneously, and this guide provides the tools to do exactly that.
- ✦Place a digital thermometer with an alarm in your tank during March and April — set the alarm threshold at 32°C so you are notified before temperatures become critical.
- ✦Mark your tank's water level with a small piece of tape each week during dry season to track evaporation rate and know when top-off is needed.
- ✦Keep a partial lid on your tank during the hottest hours of the day (noon to 4 PM) to reduce both evaporation and radiant heat absorption.
Managing Aquarium Temperature in Cambodia's Heat
The most effective tool for temperature management in a Cambodian aquarium is a clip-on cooling fan directed at the water surface. These fans work by accelerating evaporation at the surface — as water evaporates, it carries heat away with it, dropping the tank temperature by 2 to 4 degrees Celsius depending on ambient humidity. During dry season in Phnom Penh, when humidity can fall below 45%, these fans can achieve a 4 to 5 degree drop, which is often enough to bring an otherwise unmanageable 35°C tank down to a safe 30 to 31°C.
The trade-off with surface cooling fans is accelerated evaporation. A fan running on a hot dry afternoon will increase your evaporation rate from 2 to 4 liters per day to potentially 6 to 8 liters per day — requiring daily top-offs to maintain volume and prevent chemistry drift. For keepers who are home during the day, running the fan only during the hottest window (noon to 5 PM) and turning it off in the evening is an effective compromise that reduces evaporation without sacrificing the cooling benefit during the critical afternoon hours.
Air conditioning is the most effective solution for extreme heat management, but running AC continuously to keep a fish room below 28°C carries a significant electricity cost by Cambodian standards. A practical middle approach for Cambodian households that use AC intermittently is to position the aquarium in the room where AC runs most frequently — typically the bedroom or main living room — rather than in a dedicated fish room, garage, or shop front that never receives AC. The tank benefits from the AC hours without requiring dedicated running costs.
For keepers running large tanks or valuable fish during the most extreme April heat, small aquarium chillers are available through import from Thailand and are stocked by several Phnom Penh specialty shops. These units work like a small air conditioner for your tank water, maintaining a precise temperature set-point regardless of ambient conditions. They are energy-intensive to run — typically 100 to 200 watts — but for a tank holding expensive discus or sensitive marine fish, the cost is justified. The investment range in Cambodia is typically $150 to $400 USD depending on capacity.
- ✦A clip-on surface fan is the most cost-effective first tool for dry-season cooling — available at Phnom Penh aquarium shops for 15,000 to 40,000 KHR.
- ✦Run surface fans during the afternoon heat window only (noon to 5 PM) to balance cooling benefit against increased evaporation.
- ✦Position your aquarium in a room that receives AC use if possible — it costs nothing extra and significantly reduces peak temperature exposure during hot months.
Top-Off Water and Chemistry Drift: The Evaporation Problem
When water evaporates from your aquarium, only pure H2O leaves the tank — all dissolved minerals, salts, and hardness compounds remain behind. This means that if you evaporate 10 percent of your tank volume and replace it with mineral-rich Phnom Penh tap water, you are actually making the mineral concentration problem worse, not neutral. The correct approach is to top off evaporated water with very low mineral content water — either reverse osmosis (RO) filtered water, distilled water, or rainwater collected cleanly.
Phnom Penh tap water has a moderately high mineral content by tropical fishkeeping standards, with general hardness (GH) typically measuring 8 to 12 degrees dH and KH (carbonate hardness) in the 4 to 8 range depending on the district and season. During dry season, a tank that receives tap water top-offs daily rather than low-mineral water will see KH and GH rise progressively over the course of weeks. The visible result is pH creep — your tank pH slowly climbing by 0.1 to 0.3 units per week — which is stressful for soft-water fish like tetras, discus, and most South American cichlids.
The most practical solution for most Phnom Penh keepers is to use filtered water for dry-season top-offs. A basic reverse osmosis (RO) unit costs between 500,000 KHR and 1,200,000 KHR installed and produces low-mineral water suitable for both aquarium top-offs and drinking. Many Cambodian households already have these units for drinking water — using the same supply for aquarium top-offs during dry season adds negligible cost. Alternatively, the filtered drinking water sold at community water stations (those blue or yellow tanks seen throughout Phnom Penh) is typically low-mineral and works well for top-offs at roughly 400 to 800 KHR per liter.
Weekly water parameter checks during dry season are more important than during other times of year. Testing KH, pH, and general hardness once a week — rather than the once-per-month schedule adequate during stable conditions — gives you a clear picture of whether chemistry drift is occurring and how fast. A rising pH that you detect at 0.2 units above baseline is easily corrected with a water change. The same drift caught at 0.8 units above baseline, after three weeks of unchecked accumulation, requires a much larger correction that stresses your fish more significantly.
- ✦Top off evaporated water with RO, distilled, or filtered low-mineral water — not tap water — to prevent progressive mineral concentration during dry season.
- ✦Test KH and pH weekly during March and April when evaporation is highest; monthly testing is not sufficient to catch dry-season chemistry drift early.
- ✦Community water station filtered water (400 to 800 KHR per liter throughout Phnom Penh) is a convenient, affordable source of low-mineral top-off water.
Species-Specific Heat Tolerance: Which Fish Survive Cambodia's Summer
Not all tropical fish are equally equipped for Cambodia's dry-season extremes. Understanding the thermal tolerance ranges of your species is essential for deciding which fish to keep through the hottest months and which to protect with active cooling. Some fish are genuinely well-suited to Cambodian conditions year-round; others are technically tropical but have much narrower comfort windows than their "tropical fish" label suggests.
Fish that handle Cambodian dry-season temperatures well (comfortable to 33 to 35°C) include most locally adapted Cambodian livebearers (guppies, platies, mollies), common goldfish in shallow outdoor ponds, bettas, and many cichlid species from South and Central America that come from naturally warm water bodies. These fish will still benefit from cooler conditions but can tolerate extended exposure to temperatures in the low-to-mid 30s without acute stress.
Fish that are genuinely at risk during Cambodia's peak dry season (begin to stress above 30 to 31°C) include most tetra species, discus, German blue rams, corydoras catfish, and most South American dwarf cichlids. These fish come from river environments that rarely exceed 29°C naturally, and prolonged exposure above 30°C suppresses their immune systems and accelerates their metabolism to unsustainable levels. Keeping these species through March and April without at minimum a surface fan, and ideally a chiller, risks shortened lifespans and increased susceptibility to infections.
Coldwater and subtropical species — fancy goldfish varieties, koi, and white cloud mountain minnows — do poorly in Cambodia's dry season and are best avoided by Cambodian keepers entirely unless you have a heavily filtered outdoor pond where evaporative cooling keeps temperatures lower than indoor conditions. Fancy goldfish sold at Phnom Penh markets are popular but often short-lived for this reason — they are simply not matched to Cambodia's climate year-round.
- ✦Match your fish selection to your cooling capacity — if you have no surface fan or chiller, choose heat-tolerant species like bettas, mollies, and cichlids for dry-season tanks.
- ✦Plan your most expensive or sensitive fish purchases for October to February when ambient temperatures are at their annual low — not March or April.
- ✦Research the natural river temperature of your fish's origin habitat — this gives you a more accurate comfort window than the generic "tropical fish" label on shop tags.
Water Changes During Dry Season: Frequency and Method
The standard advice for aquarium water changes — 20 to 25 percent weekly — is calibrated for stable conditions. Cambodia's dry season is not stable conditions, and the water change schedule should be adjusted accordingly. During peak dry season, performing slightly larger water changes (25 to 30 percent) or slightly more frequent changes (twice weekly rather than once) helps counteract both the mineral accumulation from daily top-offs and the faster waste metabolism that comes from fish living at higher temperatures.
Water temperature matching is more critical during dry season than any other time of year. In Cambodia's warm tap water environment, the incoming water for a water change is usually within 2 to 3°C of tank temperature naturally — this is actually an advantage compared to cold-climate fishkeeping where hot water must be mixed precisely. However, on the very hottest dry-season afternoons, Phnom Penh tap water that has been sitting in exposed rooftop tanks can actually be warmer than your aquarium. A simple wrist test — placing your wrist under the tap to feel the temperature — takes two seconds and prevents a reverse thermal shock.
Use a quality dechlorinator for all water change water during dry season. Phnom Penh's water authority increases chloramine dosing during dry season in response to lower water flow and higher microbial load in the distribution system. Basic sodium thiosulfate dechlorinators neutralize chlorine but not chloramine — if you have been using a budget dechlorinator, dry season is the time to upgrade to a chloramine-neutralizing product like Seachem Prime or equivalent. A small amount of chloramine reaching your tank during a water change will cause gill irritation and immune suppression in fish already stressed by heat.
Siphon the substrate thoroughly during dry-season water changes. In warm water, organic waste decomposes faster than in cooler conditions — uneaten food, fish waste, and plant debris that would slowly break down at 26°C break down at near-toxic speed at 33°C, producing ammonia and contributing to bacterial blooms. A thorough gravel vacuum during every water change removes the fuel for this accelerated decomposition and keeps your nitrogen cycle parameters stable despite the temperature-driven metabolic acceleration.
- ✦Increase water change frequency to twice weekly during March and April, or increase volume to 30 percent, to offset faster waste accumulation and mineral drift.
- ✦Always check tap water temperature before adding it to your tank during dry season — rooftop water tanks can heat Phnom Penh tap water above ambient air temperature on hot afternoons.
- ✦Upgrade to a chloramine-neutralizing dechlorinator during dry season when Phnom Penh water authority increases chloramine dosing.
Feeding Adjustments and Monitoring During Extreme Heat
Fish metabolism accelerates with temperature — at 33°C, a fish metabolizes roughly 1.5 times faster than at 27°C. This might seem to suggest that you should feed more during hot periods, but the correct approach is actually the opposite for most species. While metabolic rate is higher, the oxygen dissolved in warm water is significantly lower, making the energy cost of processing large meals prohibitive. Overfeeding in a hot, oxygen-depleted tank produces ammonia surges faster than your biological filter can handle them and is a primary cause of dry-season fish loss.
Reduce feeding by 20 to 30 percent during the hottest weeks of dry season. Feed only once daily rather than twice, and size each feeding so that all food is consumed within 90 seconds. Remove any uneaten food immediately with a net or turkey baster. This reduction in organic load significantly reduces the ammonia and nitrate accumulation rate at a time when your filter is already working harder due to elevated temperatures accelerating the biochemistry of the nitrogen cycle.
Watch for the earliest signs of heat stress during dry-season peak weeks: fish spending more time near the surface or near any airstones or return flow, reduced appetite before you reduce their ration, color fading, and general lethargy in species that are normally active. These are not disease signs — they are behavioral thermoregulation responses. They tell you the tank is running at or above the fish's comfort ceiling and that intervention is needed. Adding a surface fan, a partial water change with cooler water, or simply reducing any lighting (which generates heat) are immediate responses.
Keep a log — even a simple notepad or phone note — of your tank temperature twice daily during March and April, noting the high of the afternoon and the overnight low. Over two to three weeks, this log tells you whether temperature fluctuation is within a safe 4°C daily range or is exceeding it in ways that indicate thermal stress. It also gives you documentation of the conditions if an unexplained disease outbreak occurs after the hot season, allowing you to understand whether thermal stress was a contributing factor.
- ✦Reduce feeding by 20 to 30 percent during peak heat weeks — metabolic acceleration in hot water means overfeeding generates dangerous ammonia faster than your filter can process.
- ✦Log tank temperature morning and evening through March and April to track daily temperature swing and identify heat stress risk before fish show visible symptoms.
- ✦Turn off or reduce aquarium lighting during the hottest hours of the day — lighting generates heat that compounds ambient temperature stress in enclosed tank environments.