Get a Shubikan gold fish, and A comet goldfish, I have that combo and they love each other in my 30g tank. But the shubikan *was* a little mean to the comet untill just recently.
don't listen to question2ask, betta fish DO NOT LIKE OTHER fish, get him some fake plants and go outside and find a decent sized rock, my fishes love real rocks.
He has an illness, did ya recently introduce a new fish to the aquarium? My old fish had that and he died the next day. you should isolate him and give him extra care, if you don't he will most likely die, but.
I got my uncle's old 30 gallon fish tank yesterday and i went to go get another fish, i currently had 1 commet gold fish that i've had for a long time, and then i went to get her a friend and i got a shubunkin gold fish, can they interbreed?