I raised tilapia this year in South Florida. They need to be kept above 55 degrees so I only added heat to the tank when the temps hovered near that temperature. We had a warmer than usual winter so I only used aquarium heaters (two of them in a 300 gallon tank) a few times. Otherwise I didn't worry about the temperatures even though the fish grow better and faster when it's very warm. The fish still did their jobs and my plants grew beautifully. In the summer time the water temperatures are in the 80s and the tilapia love that.
The heaters use a lot of energy so I was careful to only use them when necessary. I don't know if this really answers your question but this was my experience.
Up here in CT I have to heat during the winter. I heat the air and the water with a rocket mass heater and usually fire it just once a day....sometimes every few days. Usually when the water gets down to 55, I'll burn it for several hours to get the water back up into the low 60's. There's enough mass in the system so it can retain the heat for a while and the fish don't seem bothered by letting the temperature swing by the 10 degrees. By using the rocket mass heater, I can burn very efficiently and extract a higher percentage of heat from the fuel vs a propane heater. There really isn't a need to regulate the temperature precisely...it's the people that like the consistent temps....the plants and fish don't really care. ;-)
Water is hard to heat and hard to cool. Heating the atmosphere is the most inefficient way to heat water. You need a heat exchanger in the water to make it anywhere near sensible in cost. Of course, the closer the air temp matches the water temp the less heat will be lost.
So, the more efficient your structure retains heat, the fewer BTUs needed to maintain your water temp. Other than raft systems, AP beds, troughs, towers, and etc. are giant radiators constantly cooling the water. So water and air temp are pretty much hand in hand in importance concerning heat loss. Raft systems can be the best insulated systems, but aeration is critical because of the little surface of water to air interface that exists. Other than that you can insulate the system extremely well reducing heat loss. Then as long as air temps are above freezing, your system will do relatively well if provided enough light.
There are many ways to heat the water. Aquarium heaters, snorkel stoves, the Fish Sweater, solar panels, electric, gas, or biomass heated hydronic systems, rocket stoves with heat exchangers, etc. It all comes down to your region, preferred energy, creativity, and budget. The most efficient systems will have the biggest up front cost, but will save you immense amounts of money in energy costs.
Yes, my experience was that during chilly days (by our Florida Keys standards) the water running into the beds from the warm fish tank water would be cooled pretty fast. At night I would run only run my pumps for 20 minutes per hour and one especially cold night I shut them off altogether until when the sun same up. I was careful to check ammonia levels to make sure there wasn't too much buildup. I always ran the pumps during the day no matter how cool. Again, we were fortunate this year because we didn't get long cold snaps like we can occasionally. In 2010 we had a entire month of cold temps. I didn't have an aquaponics set up then so there was no issue.
Keep in mind when asking questions to let us know things like where you are located to save us looking that up. I see you are in NY where I grew up.
It all depends on what time of year, are you in a GH, and so many factors i.e. do you heat with wood now and do you have your own source and chainsaws, splitters, etc. If you don't for example heat with wood now I would not go that direction unless you have a cheap source of ready split wood. Personally I love wood heat but I am well prepared and heat 2 houses with it already and have my own trees and tools, front end loader, splitter, etc. so for me it is a no brainer. Believe me I can heat all the water you want to with wood in one of my wood stoves that heats the water and the GH at the same time.
It also depends upon the size of your system. If you are talking one 150 gal FT then there are electric heaters that will do that fine but what about the plants. Are you going to grow year round. Then you will need a well insulated GH up there in NY. If in a heated basement under grow lights the grow lights may be enough. See what I am driving at. You tell us exactly how, when, where and how big a system and we can nail down your options much better. I am at 3000 feet in the mountains of NE TN now so I chose trout and right now (May) my biggest concern is keeping the water COOL below 70. We still get nights in the 30's and that has saved our butts so far but I hope to harvest them before we get too warm.
That is the next consideration. Can you grow the catfish out in the months that don't require extra heat and switch to a colder temp fish like trout for the Winter? Take it from me cats don't like cold but trout sure do. Also remember that the bigger the system the easier it is to regulate the temp swings.
I have a fish sweater which is one week old. I realize it is not the right choice for me and want to sell it. It is set up to run propane, natural gas, solar, etc. Has a 4 foot leader hose. Obviously in great condition. Send me a message if interested.
Do you have any insulation on the fish tank to help keep it cool or warm?
If you're running the system in your basement, I probably wouldn't heat the water too much..if any. You're going to have high humidity in the basement and will increase the risk of mold/mildew in your house. The cold foundation walls during the winter will condensate a lot. Normally, I would agree with Rick, but this is one of the rare cases where I would say you should heat the air instead of the water, even though it isn't very efficient way of heating the water. A lot of this depends on how much air flow you have going through your basement.
First understand that the water temperature will try to follow the Oatside Temperature (OAT), but it can't catch up, resulting in your water tending to settle in at the mean temp, or the average of the high and low temps gives a good approximation.
The size of your tank is a factor. a larger mass will minimize the temp. variation.
My scheme is to take advantage of any solar heat during the day to get to the target temp. At night when power is cheap at least on my service, I have some 300 watt aquarium heaters to help hold the temperature up.
I also have my main FT inside of a 10' Dia Geodesic dome which is insulated to R13. This is under construction now. I am in the process of installing the insulation.
The main thing is to do what you can passively first then go to power as required.
Don't wait till fall to work this out or you will end up with a house full of aquariums as winter lodging for your fish.