I feel for ya, bad luck. I know about those damn fungal infections from my pet fish. Damn ick errrrr. I think you need to get a bigger aquarium if you get the chance. 20gal sounds alittle small to me anyway. I know a petshop that sells large ones from 60-100 bucks and they are like 50 gal or more. I really think it would be a good investment. Then use your current pump and filter but buy another one too from wally world or something. And deffinetely get a water temp gauge so you can be more ontop of the spawning and stuff. I know you know about that stuff and I'm sure it must be incredibly related. Maybe changing the location of the fish tank to different places in your house could be used to regulate the temperature in all the seasons Put them somewhere warmer in warm months and cooler in cool months, if you don't use heat or ac much then just ignore that.
I don't know how much experience you have with keeping those types of fish caus god knows I have none at all but want to do it one day. Maybe you could call a BPS or 2 and talk to the aquarium keeper. Free advice from someone that has to deal with your problems to the extreme?
p.s keep your eye on the other fish because they could be spawning too. The white crappie atleast and the catfish very soon.