Question
During the peak of last summer we had several goldfish that died after several days of hot weather and I suspect it was related. According to the Goldfish article on Wikipedia:
Extremely high temperatures (over 30°C (86°F) can also harm goldfish
The ambient temperature sat over over 40°C (104°F) during the day and only dropped to a little under 30°C (86°F) overnight so it's almost certain the tank temperature would have exceeded that amount.
Doing some research I see there are some commercial tank cooling products available although they are relatively large / expensive and this is only a once or twice a year event in my area. A few thoughts I'd considered are:
Setup a fan pointing across the tank as a form of evaporative cooling. I assume though that would cool the top layer of water more than the bottom and I wasn't sure if the differential in temperature may make things even worse?
Introduce a large amount of tap water that I measured as a bit under 20°C (68°F) because it comes from a cooler mountain range. Considering that would need to be treated quickly chemically I could see that also having potential downsides.
Add ice cubes that have already been treated, but I imagine that could result in them floating on top and mainly cooling the top portion of the tank.
I wondered if anyone has been in a similar position and can recommend a good alternative or suggest expert opinion on which of the above options may be best?
Answer
Well, you didn't say so, but I assume you have a thermometer inside your tank, don't you? Or did you measure the temperature using a portable, temporary one? Since your tank is inside your house, it's not directly at sunlight, perhaps some breeze was on it, etc.
For the solutions, since it only happens few times a year, I'd go with ice cubes of pre-treated water, or as Baarn suggested, water that you have taken from your aquarium (although it might get... smelly... when you freeze and unfreeze it, since you´ll be killing some bacteria). Since they won't have chemicals that are harmful for your fish, it can be used without problems.
Since some energy input is required to transform ice to water, the ice will "steal" this energy from the water, cooling all the tank. And due to convection, the cold water around the ice cube will flow to the bottom of the tank, and the warmer water will flow to the top, get in contact with the ice cube, and doing a cycle.
You'd just need to make things slow (so you won't be changing the water temperature too fast, causing some cold shock on your fish). The thermometer will be your friend here.
Answered By - woliveirajr