07/31/2024
This time of year I get a LOT of questions regarding how to keep an axolotl tank cool. I usually start by explaining that, as long as the tank is in an air-conditioned space set at or below 71 F (which is typical of homes in the USA), you have little to worry about.
I made the mistake of commenting this (or similar) on a post to someone who was very concerned after being told that her axolotl would die in a room at typical room temperature, and was immediately flamed by all of the axolotl Karens with too much idle time. Well, ok… Not ALL of them. That would crash the internet.
So let me explain why a tank kept in typical room temperature is fine, and how and when to keep things cooler when it is really needed.
Axolotls seem happiest around 65 F (18 C), and tolerate gradual water temperature fluctuations from around 45 F to 74 F (7 C to 23 C) without any health issues. Whenever I say this, there are always a lot of objections and arguments, and some of them are valid. Let's keep in mind that Lake Xochimilco is a sunlit, high-altitude lake (originally) fed from snow run-off and artesian springs. The surface water is heated to 75 F (24 C) and higher on still days in the Summer sun. Axolotls avoid the hot surface water, staying below the thermocline most of the time - but do surface now and then to catch floating prey, gulp air, and lay eggs on shallow water plants. In fact, axolotls eggs actually have higher hatch rates in warmer water (up to 74 F). The bottom of the lake is always around 60 F (16 C) in the Summer and drops below 45 F (and sometimes lower) in the Winter. That 45 F, by the way, is the temperature of the entire lake, top to bottom, in Winter, so clearly axolotls can survive long periods at that temperature - but they don’t eat and are mostly immobile until the water warms up above 50 F..
Edit for clarity: As stated, axolotls sometimes come above the thermocline. They do not stay there! Long term exposure of juveniles and adults to temperatures in excess of 70 F reduces immune function and promotes infection. That is why we need to know how to reduce temperature in emergency situations.
If there is nothing actively heating the tank (like a faulty filter motor or inappropriate light source) the temperature of the water in your tank is lower than the temperature of the air in the room the tank is in.
This is due to the process of evaporative cooling. Here’s how it works:
Water is a liquid at room temperature (citation needed), which means that the molecules of water are close together, but not tightly connected. They are a bit “sticky”, but are mostly free to slide around each other. Molecules are always moving; some faster and some slower. In fact, water temperature is really a measure of the average speed - more precisely, the kinetic energy - of those water molecules. In a cold tank the molecules are moving slowly compared to those in a tank of hot water (which does not mean they are really all that slow. The average speed of water molecules at room temperature is around 1,300 mph (590 meters per second), which is faster than the speed of sound in air).
Since temperature is a measure of the AVERAGE kinetic energy of molecules that are bouncing off one another, that means at any given instant there are some molecules that are moving very slowly and some that are moving very quickly. Molecules beneath the surface of the tank that are going very fast soon hit a slower molecule and slow down again. At the surface of the water, though, if a molecule is moving fast enough to overcome the “stickiness” (adhesion) of the other molecules it can fly out of the water and into the air! It is these fast-moving molecules escaping from the water that we call “evaporation”.
Of course, air is made out of molecules, too, and they’re heavier than water molecules. Many of the molecules that escape collide with air molecules and bounce right back into the water. The air molecules, though, are about ten times farther apart than the molecules of liquid water. This leaves a lot of space between them for the escaping water molecules to travel before they hit an air molecule. If the water molecule gets far enough from the surface of the water, it is unlikely to ever come back, so there is then less water in the tank, and more water in the air..
The fast-moving molecules escape from the surface, leaving the slower molecules behind. Remember that fast-moving molecules = hot, and slow-moving molecules = cold. As fast molecules escape the water, the temperature of the water drops lower. The more fast-moving molecules escape, the cooler the water gets. This is Evaporative Cooling.
Due to evaporative cooling alone under normal conditions in an open aquarium you can expect the water temperature in the tank to be 2-3 degrees Fahrenheit (1-2C) cooler than the ambient air temperature.
But there are several things that can increase or decrease the effectiveness of evaporative cooling. Anything that increases the rate of evaporation also increases cooling. Anything that reduces evaporation reduces cooling.
Lids:
If you have a lid on the aquarium, there is no evaporative cooling at all. This is because any fast-moving water molecules that escape from the surface of the water have nowhere to go. They bounce around in the space under the lid for a bit, then end up right back in the tank water. Any thermal energy they took with them when they escaped goes right back into the water, too.
In fact, having a lid on your aquarium will actually result in the tank getting a bit hotter than the ambient temperature of the surrounding air! Even though axolotls are “cold-blooded”, they do, in fact, generate some heat. Every chemical reaction in their bodies releases a bit of energy as heat, warming them and the surrounding water. The nitrifying bacteria in the filter and substrate also generate heat as they break down ammonia and nitrite.
If you have a submersible flow-filter in the tank, that can generate a great deal of heat! I actually use such a submersible filter in my guppy breeding tank to keep them warm as well as filter the water.
I understand that it is often absolutely necessary to cover your aquarium - especially if you have cats! In that case, use a wire screen cover that is strong enough to protect the axolotls from curious felines. Don’t use a window-screen mesh. The fine mesh of such a screen reduces evaporation almost as much as a solid lid.
Humidity:
The air can only hold so much water. The amount of water in the air expressed as a percent of the total amount that the air can hold is Relative Humidity (RH). In other words, a RH of 50% means that the air is holding half of the water it can possibly hold. At 100% RH the air is as full of water as it can get. This means that evaporative cooling does not work at all at 100% RH!
85 F feels really hot in Florida, but not so much in Arizona. The RH in Tampa Florida on a typical Summer afternoon is high, around 80%, so water molecules escaping from the surface of your skin have nowhere to go, and just bounce back. In AZ the RH is usually under 10%, so water molecules evaporating from your skin easily escape and are carried away with the breeze, which cools your skin.
If the humidity in your home is very high, as is often the case in tropical climates and/or rainy weather in the Summer, evaporative cooling may not do much at all - even with a fan!
Fans:
A fan blowing air across the surface of the water will cool the tank a great deal more than still air. Remember that evaporative cooling requires fast-moving molecules to escape from the surface of the water, but if it bounces back into the water it doesn’t cool any at all. By moving air across the surface of the water, the fan blows any escaping water molecules away before they have a chance to bounce back into the water.
Note: Just aiming a fan at the side of the tank does almost nothing to cool it off! The air must blow across the surface of the water to assist evaporative cooling.
Using a fan for evaporative cooling can reduce the water temperature by about 4 to 7 degrees Fahrenheit (2 to 4 degrees Celsius). In optimal conditions, with low relative humidity and a powerful fan producing a lot of air flow, the temperature drop can be even greater.
Suppose you have a 25 gallon (∼100-liter) aquarium with a surface area of 780 square inches (0.5 square meter) and an ambient room temperature of 72 F (22 C) with 40% relative humidity. Using a fan, you might see the water temperature drop from 70 F (21 C) to around 65 F (18 C) over several hours. This is a rough estimate, and actual results can vary based on the factors mentioned above.
Replacing evaporated water:
Evaporation removes water molecules from the tank. That water has to be replaced. Note that evaporation ONLY removes water - It does not remove any minerals dissolved in the tank water. This means that the concentration of dissolved minerals, ie; water hardness, increases as water evaporates from the aquarium. If you use tap water to replace water lost to evaporation, you are introducing even more dissolved minerals into the tank.
Typically, water is described as 'hard' if the General Hardness (GH) is > 16° (285 ppm), and 'soft' if it is < 8° (142 ppm). Since the axolotl's natural environment is (was) supplied from springs and mountain snowmelt, they do best in moderately-hard water ranging from 7–14° (125-250 ppm).
Hard water causes problems: Hard water tends to leave deposits - mostly calcium and magnesium carbonates - on your aquarium glass and anything else at the surface of the water where evaporation leaves the minerals behind. In very hard water bacteria can form adhesive mats on the skin and gills of an axolotl, increasing risk of infection. It is even possible to get calcium deposits in internal organs, similar to kidney stones in humans.
That is why you must always replace lost water with distilled water (which is pure water collected by heating it to a boil, then cooling the steam to collect pure water) or with R/O water (which is pure water produced by removing all dissolved materials using reverse osmosis; basically forcing water through a very fine filter membrane under high pressure).Topping off the tank regularly with distilled or R/O water to replace water lost to evaporation maintains stable water chemistry.
You could also replace most or all of the tank water now and then with new, treated tap water - but as anyone with experience with keeping aquariums knows, this is very likely to crash the nitrogen cycle of the tank.
So what do you do if your home is too hot and humidity is too high?
If the situation is temporary, anything that cools the water down to safe levels until the danger is over will do.
If it’s a regular or permanent situation, either buy a good aquarium chiller or give the axolotls to someone who has a proper environment for them!
In an emergency, you can simply add ice to the tank. You may worry that the melting ice might be treated with chlorine, and you’d be right. Adding a few drops of a dechlorinator (dosing to approximately match the volume of the ice) solves that problem.
You may also worry that adding ice to the water will cool it too quickly. That’s not a problem as long as the volume of ice added is much smaller than the volume of the tank. Water is very resistant to temperature change (we physical chemists say it has a high specific heat). As the ice melts and then warms up a bit, the cold water will sink to the bottom of the tank, blending in as it goes. The overall temperature change is gradual, and the axolotls do just fine adjusting to the change - now and then! Repeated forced adjustment to changing temperature is stressful, and will eventually cause health problems. If you have to do this often, it is time to find another solution.
You can remove some of the tank water and replace it with colder water. This is a bit risky, though. (1) Sudden temperature change can be too much for the axolotls, and (2) The sudden change of water chemistry can kill your nitrifying bacteria. Always add the cold water gradually over a period of time rather than all at once, and, of course, always use water that has been treated to remove chlorine and other contaminants.
Again, adding cold water or ice to the aquarium is a TEMPORARY, EMERGENCY procedure! It is not a long-term solution. If high water temperature is a problem, and you can’t just set your air conditioner to a lower setting, an aquarium chiller is needed.
Hope this helps.
