Pea puffer? ?

I have them, and I'd skip them for this tank for several reasons.

- Pea Puffers are still rather new to the hobby and that's why information about them is contradictory. 

- Currently i have only one male, which seems to be a mistake, but hey, that was consistently recommended for my tank size. (Now i'm thinking how i can get him some playmates without a carnage... and possibly a larger tank.)

- They are very intelligent, curious and aggressive. I wouldn't trust them with shrimp, unless they are intended as food. I wouldn't trust them with other fish either as they can get very nippy.

- You need to handle their intelligence and create a rich environment for them. Without plenty of environmental enrichment they can get bored and develop repitive behaviors like glass surfing. 

- Live plants are nice, but so are different colors (think Cryptocorynes), objects and textures. E.g. my lone male is in a 8 gallon setup: 3 seiryu stones, 3 different types of Crypotorynes (no id), a bunch of Rotala, spider wood with Anubias barteri var. nana planted on it (regular green and gold variety too) and some floating plants. If you are keeping multiples, you need to structure the tank in a ways that they each can get away from each other.

- They have a preference for LIVE or FROZEN food. No freeze dried or ready made BS. They need a variety of foods, they help to enrich their environment. And hunting is fun. They prefer 2 small feedings a day. (Btw, they are messy eaters. They chomp off the head of snails but rarely eat the entire body.)

- Mine is in the kitchen / dining room area, which gets a lot of traffic, so he has plenty of things to inspect outside his tank.

- Mine shares a tank with a bunch of neocaridina shrimp I intended to cull. Some are now larger than him, some are eaten (which is rather messy and gruesome). I rarely find young ones, so I assume he eats most of them.

- They are scalless fish. Which puts them in the same sensitivity levels as loaches, medication-wise too.

- Despite the common belief I found them rather easy to sex. Mature males have very yellow belly, with a brown stripe when in breeding condition and iridescent, neon blue stripes around their eyes. The females have lots of tiny spots on their back, which the males lack completely.

- That being said: the safety and well being of your current fish is most important. As I said, I wouldn't try incorporating dwarf puffers in ANY community tank. If you do, you need to have another tank ready to separate them. Combine this with their needs of a lushly planted, well structured tank, it's easier and safer to start them in their own, species only setup.

- As far as the well being of your fish is concerned: both the otos and the pygmy cories would do better in a larger shoal (unless you are phasing them out, in that case, you are fine). The general rule of schooling / shoaling fish is the "minimum 5 or 6 / species but the more the merrier." Otocincluses are no exceptions. However TINY fish, such as Pygmy Cories (Ember Tetras, Espei/Hengeli Rasboras, Boraras, Celestichthys etc.) ARE an exceptions: because of their numbers, they need larger numbers to feel really safe.

So what I'd do...:

- Bump of the numbers of your current fish: 6 Otos, 12 Pygmy puffers

- I'd either add another species of tiny shoalers: Ember Tetras (if you want to keep with the South American theme), Espei/Hengeli Rasboras or Boraras if you want to go Asian (Celestichthys are not recommended, they tend to use the bottom half of the tank and you already have enough bottom dwellers)

- OR get another Betta / Dwarf Goraumi / Honey Gouramis. Pygmy Gouramis are known shrimp hunters, they might or might not leave your Amanos alone (btw, they are noisy, don't be surprise if you decide on them). I'm a big fan of Honey Gouramis as they are rather social and docile. You should be able to keep a male and a harem of 2-3 females in a well planted, well structured 20 gallon tank without problems. In my experience they leave shrimp alone. (Gouramis, just like Bettas are top dwellers, plants that touch the surface and floating plants are needed in their tanks.)

Pea Puffers are NOT community fish-they would just kill and eat everyone else in the tank. Don't go there. 

Pea Puffers are one of the most aggressive fish you can get. They would kill your shrimp and Corycats. They are horrendous bullies and fin nippers, and can kill fish much larger than themselves. Pea Puffers should never be kept in communities. If you want Puffers they would need their own tank.

If you are looking to branch out, there are other options.

A Gardneri Killifish would make a nice center piece. You can keep a single male or a mated pair, but females aren't as attractive. Like Bettas, Killifish are often kept in small unfiltered tanks, and so if you got one you would be giving it an especially good home. Not all stores carry them, but if you call around you should find someone that has them.

Gourami make good centerpiece fish. Dwarf Gourami, Honey Gourami, or Sparkling Gourami would all work for your tank. You should choose 1 species though, and not keep them with Betta as they are so closely related they fight. Dwarf Gourami work best as a single specimen. Honey Gourami are more social, and can be kept singly, in pairs, or in trios as long as there is only one male. Sparkling Gourami are very social, and need at least 3 of their own kind, and multiple males can live together but it is best to have 2 females per male.

3 male Platies would make a nice addition too, and come in a lot of color and fin varieties. 3 is their minimum group number. They don't school, but they do like each other's company. Females will usually overstock you (they store sperm and are almost always already pregnant when bought), so I recommend all males. These guys do need a pH of 7.0 or higher, but are otherwise quite hardy.

Some people have successfully kept pea puffers with other fish and in other cases the pea puffers eat small fish. They will eat shrimp too if they get ahold of them. Since your tank is heavily planted you can try it if you want but there is no guarantee that it will work out. 

As long as you don't have snails in there you should be good, because if they are viewed as pet snails to you... they sure aren't gonna be for the puffers. Puffers love to pick at them or worse... eat them. Now with that being said... each puffer has a different personality and one may like to nip tails of other fish and one may not. It all depends, but since you will have three they may be too occupied with each other and won't bother the other fish too much except to hover around them to sniff 'em out and move on to something else. They're curious little fish... and always exploring their surroundings and sometimes getting into trouble but harmless. You will love 'em I promise. 

Keep in mind... some pea puffers may NOT eat fish flakes or pellets. Mine only eat two things... and that's small snails and frozen blood worms.