Insulate the base, back and sides of the tank with 1 to 2 inch thick polystyrene foam sheets. Just tape them to the outside of the tank.
Have a coverglass on the tank. Use 4, 5 or 6mm thick glass for the covers because they are less likely to chip or crack than thinner glass, and the thicker glass holds heat better. Most coverglass sold at pet shops are 2 or 3mm thick so you might need to order thicker coverglass in or talk to a glazier.
Have coverglass well fitted so virtually the entire top of the tank is covered. A lot of shops have big corners cut out for aiirlines and heater cabless, etc. Most of these corners don't need to be as big as they are. I used to cut corners off that were less than 1 inch at the widest part and most were about 1/2 inch.
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Use big light units to cover several tanks. I had a number of tanks next to each other and ran 4 foot light units over them. I used fewer power points and had better lighting over the tanks compared to running smaller lights on individual tanks.
You can also run a couple of fluorescent or LED lights in the room and not have lights on the tanks. However, if you are growing live plants in the tank, then they will need a light above the tank.
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If you have lots of fish tanks, put them in the same room so they lose heat in the same room. The water holds heat better than air and if you have a number of tanks near each other, they help to warm the air around themselves and the other tanks. This reduces heating costs compared to having one tank in each room..
If you have a door on the room the tanks are in, try to keep it closed when the weather is cooler.
Close the windows and curtains too if it's cold outside. Most places in the USA and UK have double or triple glazed windows now so heat loss through windows is not much of an issue. But in places like Australia, the government is unwilling to enact laws about houses being made energy efficient. Yes they must have insulation in the roof and around the hot water pipe, but that is it. ANd lots of new places don't have insulation even though they are meant too. And nobody in the government seems to care about that. What's the point of having building codes and laws if nobody enforces them?
Australia, the backwards country where common sense is sadly lacking in our political puppets, I mean leaders.
If you don't have double glazed windows, get a sheet of 4 inch thick polystyrene foam and cut that to fit in the window. Put it in the window at night and take it out during the day, assuming the weather is warm enough.
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If you have lots of tanks, maybe build a dedicated fish room with skylights and insulation in the roof, walls and floor. There are rubber matts made from recycled car tyres and these are readily available now. They come in a couple of different thicknesses and a range of colours. They are water proof, easy to lay down, can be taken outside and hosed off, and last forever. We put these in our games room, which had a concrete flood, and by the time we had done half the floor, there was a noticeable difference in the room temperature.
If you use the rubber matts made from recycled tyres, get the thicker matts (about 1 inch thick) and lay them out first and leave them for 24 hours so they can shrink back to normal size. Then cut them to fit. The matts squash out when stacked in the shop and they can shrink an inch after they have been put down individually. If you lay them and cut them straight away, you will have big gaps around them the following day. So lay them out, wait 24 hours, then push them together and cut them to fit.
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I set the heaters on most of my tanks at 18C. In spring and summer the water temperature would naturally rise up to and above 30C. In winter the temperature would drop to 18C. This variation in temperature was fine for the fish because it happened slowly over a course of months. It also gave fish like barbs, tetras and catfish a cooling off period, and in spring they would colour up and breed like crazy rabbits.