Okay, I got a 20 gallon rectangular tank with a Marineland Penguin 150 Biowheel Powerfilter, a Stealth Pro 100 heater, a hood with a flourescent light, and a net. I'm planning on setting up a tropical community aquarium and I'm wondering what other equipment I'm going to need like test kits and...
Sounds like something's wrong with his swim bladder, that's probably why he floats on his side. Did you by any chance clean his tank or any of his equipment with dish soap? You situation is exactly what happened to one of my bettas. I got some bad advice to wash his tank with soap and the next...
I would certainly NOT put any goldfish in a tank that small, especially not for a five-year-old. Goldfish produce a lot of waste, so the water will have to be changed very often. A betta will work well in that size tank and you'll have to do water changes maybe once per week.