M&M is absolutely correct: A 5.5 gallon is too small for any fish except a Betta as Otos and other Micro/Nano fish need a minimum
of six for optimum health. People do it all of the time but IMO it's not fair to schooling fish to have fewer than the minimum any more than it's fair to put a Betta in a half-gallon aquarium.
You could try Ghost Shrimp as long as the tank is densely planted (as in no wide open spaces
) so they have places to go to avoid your Betta. He may appear to be chill but he's alone; he may surprise you if you add some other critter. Do have a backup plan if it doesn't work.
As far as snails, I wouldn't recommend Nerites as they need natural algae and are very picky eaters. Most won't eat algae wafers or touch vegetables. But if you do decide to try them (I love mine), only get one; two at the most in a 5.5. Fill a small container with driftwood, rocks or marbles and place in a window sill. This will grow the algae you need for your Nerites.
African Dwarf Frogs are great and are not aggressive towards Bettas at all. Unfortunately, they take the hit for the extremely aggressive and much larger African Clawed Frogs. ACF are often sold as ADF so make sure you know the difference. Since froggers recommend one per gallon, you could easily have two or three ADF with your Betta. Actually, many frog sites recommend Bettas as the best fish tankmates for ADF.
The special care ADFs need is make sure they get enough food. Since they are almost totally blind, they hunt/feed by smell. (If they grab a Betta it's purely by accident.) Because the fish would grab their food from them, I target feed Esmeralda and Clayton frozen bloodworms with 12" long planting tongs. I alternate with HBH Frog and Tadpole bites which can be dropped to the substrate since the fish can't steal them.
Now the 10 gallon....there are some possibilities.
Just make sure you doubly clean it to get 100% of residual Gerbil urine and feces residue.
You could have a seven or more Neon Tetras or Rasboras, male Endlers or Guppies (it's a myth that Bettas see colorful male Guppies as another Betta). Since it's your first try at fish keeping, I would wait to get anything like Pygmy Cories.
Here's a really good site with all the information you need on Micro fish possibilities. I trust this site over others as Rachel O'Leary is a nationally-recognized authority on Micro fish and frequently lectures at fish keeping conventions. Hover over the photo; the little square with the arrow is where you'll find the information you need.
Freshwater Fish | Invertebrates by Msjinkzd
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