20gallon should be fine for him alone, it is pretty much considered minimum. If you can get him a little bigger tank down the road it won't hurt, but he will be okay in a 20.
Best staple diet for him is a good quality pellet. New Life Spectrum is considered the highest grade of fish food ingredient wise, though Hikari and plenty of other brands will be fine to feed him as well. Occasionally, he would probably enjoy some frozen or freeze dried protein-rich food like krill or bloodworms as a treat.
Convicts are very cool little fish, good luck with him