I have a dwarf frog with my betta. I have a hard time getting a betta comfortable in a large tank. They go insane when i put them in a 10g, even a 5g! I'm having success with 2.5g.ANYWAYS, be warned if you're getting ottos, they need a mature tank and take your time acclimating them, I hear the drip method

works best. Pick out an otto that looks kind of fat, cause that's a healthy otto. as for food, don't bother with algae discs, they wont touch them. Feed them zucchini or spinach or grow your own algae. Once you get past the first month, they're actually reasonably hardy fish. Good luck.