My betta is in a 10 gallon with zebra danios, harlequin rasboras, an otocinclus, and soon pygmy corys. Everyone gets along well. I have heard white clouds get along well, too. As was previously mentioned, there are several species of fish that get along well with Bettas... no long finned fish, no fish related to bettas, no fin nippers (for example, tiger barbs). It's good to do research before adding fish to tanks so that fish don't get nipped or die unnecessarily.