There are many reasons why you may have to cool down the temperature of the water in your freshwater tank. For example, living in Florida and having the air conditioner blow might make the water temperature skyrocket. Also, aquarium water heaters sometimes go crazy, especially when they are at the end of their “lives”. This can wind up giving your fish heat stress that will eventually kill them unless you can cool the water back down. The key to doing this successfully is by doing it gradually.
Signs Of Heat Stress
It’s always good to have a stick on or floating thermometer to keep an eye on the water temperature of your freshwater tank. However, your fish will also let you know if they are stifling hot. They will act very peculiar, whether swimming more than usual or hanging out at the bottom of the tank, gasping. Gulping oxygen from the top of the tank is not a good sign, either. A too hot tank does not have much oxygen in it.
Gradual Water Change
A good way to help cool down your tank water is by doing two things a partial water change and floating a hot water bottle full of cold water inside the tank. You don’t want to just chuck ice cubes into the tank, as this will make the temperature change too rapidly. Fish will die or be more prone to getting sick when the water temperature changes suddenly (up or down) than when you do it gradually.
A partial water change of one-fifth to one-quarter of the total tank water can help make such a gradual change. When you replace the water, you want cool or tepid water, not ice cold water. Again, we want to avoid any sudden changes.
While you’re doing the water change, you also want to increase the aeration of your tank. This is easy if you have an adjustable air pump. You simply turn it to the highest setting. But if that isn’t an option, then you need to put another airstone in the tank and get those bubbles going. Remember the hotter the tank, the lower the oxygen content.
Don’t turn the air on so high that the waves created threaten to fling your fish out of the tank. You also don’t want them to struggle hard trying to fight the new current. Some fish, like bettas (Siamese Fighting Fish) and bubble-eyed goldfish are very poor swimmers. They are used to having not much aeration to begin with. Use your judgment and the species swimming talents (or lack thereof) to help you determine how high and how long to keep the extra aeration going.
“Freshwater Aquariums for Dummies.” Maddy Hargrove & Mic Hargrove. For Dummies; 2006.
“Freshwater Aquarium Problem Solver.” David. E. Boruchowitz. TFH; 2006.
“The Everything Aquarium Book.” Frank Indiviglio. Adams Media: 2006.
Author has also kept freshwater fish for over 20 years.