We really want to get tropical fish but there's a problem. In the state we live in, it can get as high as low 40's °C in the summer. And worse, we don't have insulation installed in our house.
Would it get way too hot for the fish even without a heater?
When it gets too hot, could I even get a tank cooler for it?
Are fish tank coolers adjustable for temp?
Would it be possible without frying the fish?
Use a fan or squirrel cage blower to blow air onto the water surface. This will cool the water in the tank down, albeit at the cost of evaporation. You will have to top up the water at regular intervals, and also, this does not work well in extremely humid weather. But once you get it setup, the water will be cool as heaven for the fish.
There are coolers for aquariums, but I would spend the money on a home air conditioner.
The labyrinth fish like Bettas can easily take an air temperature of 35 °C; I expect the water to be somewhat cooler due to evaporative cooling. I keep paradise fish outside (in the shade) at 35 °C air temperature. With good aeration many tropical fish should be good to 35 °C air temperature. I keep danios, mollies and koi, under these conditions.
Tropical fish thrive between 25 °C and 27 °C. They can tolerate lower and higher temperatures, but 40 °C would kill them in short time. Aquarium chillers do exist and are adjustable for temperature. There are also some homemade designs out there. Here is one I found (on thespruce.com).
Good luck - I hope you can set up an aquarium! They are really rewarding.
Saltwater fish are very sensitive to temperature fluctuations, and can not tolerate excessively warm water. The larger your tank, the easier it will be to maintain a consistent temperature. Fans can help to lower the temperature a bit, but the best bet is to use a dedicated water chiller. These use a refrigerant, and have coils that drop into the filter box. I’d recommend looking on eBay or Craigslist to find one.
https://www.marinedepot.com/Chillers_for_Saltwater_Aquariums-FICH-ct.html
