#1. Reduce the light
Light means heat. Turn off the tank lights and close curtains and doors. If you must have the light on for plant growth, use a timer and put the light on during the cool of the night. Keep the rest of the room as dark as possible (still allowing for ventilation). Make sure your Betta bowl or small tank is not in direct sunlight. Use a reflector made of foil wrapped cardboard to reflect direct sunlight away from the tank.
An increase in water temperature means a decrease in the level of oxygen available to the fish. Crank up the air stones and air driven filters. Increased aeration will also increase evaporation, which will take some of the heat out as well.
Small changes with cooler (dechlorinated!) water throughout the day will help keep the ammonia levels down, (ammonia becomes toxic at very low levels when the temperature increases) reduce the temperature and increase the oxygen levels. Particularly important for goldfish.
As water evaporates it takes heat with it. Remove cover lids and hoods. If you have fish that are likely to jump, use mesh screens or lace doilies (with weighted corners) to stop your fish escaping.
Place a fan near the tank and allow the air to flow over the water surface. If you don't wish to remove the lids you can place a damp towel over the tank and let the evaporation cool the glass. Remember to dampen the towel regularly.
Wrap a damp towel around the base of bowls and small tanks.
For insulated fish rooms/sheds - open up the room for a couple of hours during the cool of night. Keep doors and windows closed during the heat of the day.
#2. Increase Air Flow
Keeping fish cool in Summer.
The Australian Summer can be devastating for fish keepers. Wilding fluctuating temperatures, record highs, storms and stifling humidity. Hard on pets as well as people. Here are some tips for keeping your finned friends safe and happy during those scorching Summer days.
#3. Water Changes
Remove the labels from soft-drink bottles and carefully wash and rinse. 3/4 fill with water. Freeze with the cap off (to allow for expansion). Replace the cap and float in your tank. You will need to replace the bottles regularly. Make sure you closely monitor the temperature of the tank. Large fluctuations are stressful to the fish and counter productive.
If you have an external canister filter you can stand it in a large bucket and fill the space around it with ice.
For small bowl/tanks use an ice-cube or two of pure water.
#6. Increase Water Volume
Not always possible, but something to consider when purchasing your next tank. Larger volumes of water take longer to heat up, and generally provide a more stable environment for your fish. Always purchase the largest tank your budget will allow :)
#7. Commercial Chillers
For marine and shrimp tanks, these are a must. Chillers range from "all singing, all dancing" titanium units that will set you back nearly $4000 to mini-chiller units that can cool a 30L-60 tank. If you are handy you can even make your own budget chiller from computer parts and a bit of hardware.