Grow a type of fish that doesn't need really warm water and you should be ok with 50 F though things will grow slow during winter since most fish don't eat much when the water is cool. Unless you grow trout but a 100 gallon tank is probably too small for them.
I know of some people on this forum who use HID grow lights using the "old style" magnetic ballasts and have placed the ballast inside a heavy duty garbage bag, sealed it, then dropped it in the fish tank to use the extra free heat. Since those ballasts are only 85% efficient the rest is given off as heat. I have many such ballasts (mostly 400Watt and 250Watters) and can attest to them getting, what can only be described as "really friggin hot"! Though this technique would not be practical for my set up, it may be of interest to someone else.