So big I can barely even say it! lol Really...no point in having a common goldfish if you don't have a pond. I would take it to a local fish store, they will prob take it and adopt it out for you. A 10 gal tank is not a home for a common goldie. I'm not an expert on them but I think you're supposed to have 125 gals for a pair. I mean they are seriously pond fish. And they produce so much ammonia. In an unfiltered bowl I'm shocked he's lasted this long. If you can part with the goldie, you have the makings of a nice home for your betta. A heated, filtered, 10 gal tank is an ideal home for a betta. What you mentioned before, about your last betta and the current..that is an issue sometimes. I have 10 bettas and this is what I've done with all their tanks and it works perfectly, cheap and easy fix!
Filter Currents: A Betta's Worst Nightmare With your betta in the 10 gal you could definitely add a couple large-ish nerite snails or a mystery/apple snail. I find even with my one male who has gotten rough with smaller snails, that he can't really damage the larger ones. Some people add some other tankmates with them as well in a 10 gal but I'm too nervous about that to try. I've tried shrimp but they became snacks