Ryan- #1 What size heater will we need to keep the water temp. at 75 deg.? If the temperature in the greenhouse stays above 65 I think you could get by with 10,000 watts although 3-5 watts per gallon is recommended with more volatile temperature fluctuation. 3 Given the size scenario in #2 will filtration be need for 400 fish in a 4000 gal. tank if aerated? Yes a good pump and filtration is mandatory to keep the ammonia levels in control. The answer as to size of the filter depends on how much you plan to feed. Question #2 also depends on how much food and filtration you will use. About 6% of the food you add will end up as nitrogenous waste in a few hours. The amount of ammonia produced by 400 active fish will be enormous. If there is any way you could get 40 3-4 inch fish they will produce a lot of fry and fingerling's in their long overwinter. If you want my opinion, you would be much better off ordering from the nearest hatchery that will provide them when your water temperature hits 60 degrees. This is a huge project to keep that many fish healthy for 7 months of the year. You will need to learn every aspect of aquarium keeping: Preparing your filter to receive the huge amount of fish by adding ammonia and niter bacteria to the aquarium before they arrive, Monthly 25% water changes to keep the nitrates in check, chemical filtration, fresh water for evaporation losses, aeration, testing, backup pumps, diseases-- just to name a few of the problems you will face.
