Hi Jackalope, ...I too, have been thinking about and checking out heaters. I'll share what I read at some of the different sites.
The recommendations I read are, 3 to 5 watts per gallon. If the area is cooler, stay in the higher range ( they gave the example of aquariums in a basement).

At that rate, 500 gals would take (at least) 3 of the bucket heaters. Then, if your circulating the water...you would lose lots af heat... Maybe, put a heater in the sump to warm the water before going back to the FT (?)

I'm building a 500 gallon tank outside. I'm putting 3/4 inch styrofoam between the frame and the liner. I was more concerned about keeping it 'cooler' in the summer. Now, I'll make sure I have a styrofoam lid too (for winter, when needed)

Could you wrap the sides of your glass tanks with styro, to help keep in some heat ? Maybe even set the tanks on some styro ? I think I remember Stucco was talking with someone about having a heavy load sitting on it and the styro holding the load (?) I remember a picture of a pallet of tile/stucco mix sitting on styrofoam(?)

I went in a local pet store looking/ pricing some heaters. The owner warned me, when I did get one...(everyone is sold out of them here)..let it sit in the water for 15 mins or more BEFORE plugging it in and turning it on. He had several people cracking the glass by not doing it.