Thanks for the quick reply! (And the nice welcome.) So far the crab and betta have been okay. Two far apart worlds. I might need to get some places for the crab to hide?
Hmm... So you think a school of something in the tetra family won't work? I have been reading about the downsides of neons and thinking perhaps the warmer-preferring neon *cardinals* or the fun-schooling lemon tetras -- if I get enough of them (6+). Will that curb the nippiness? The lady at the shop had suggested tetras & neons for coinhabitents since my tank is 16 Gal.
Also/or... maybe shrimp of various colors?
You are getting extremely bad advice from that shop. I'll go into this further to explain.
The problem is the behaviour tendency of the various fish. These conditions are programmed into the species DNA.
Tetras (and this applies to almost any smallish shoaling fish, like rasbora, barbs, danios as well) are generally peaceful, though some species are exceptions. They live in large groups, and need a decent number to interact. Bettas are solitary fish, and they are very sedate. They also have very long flowing fins.
In sufficiently large tanks, shoaling fish may work with less active and sedate fish. In smaller tanks, this almost never works. Even ordinarily very peaceful fish, like the neons, can suddenly become quite nippy (consider it play in fish terms) when they are confronted with a temptation such as the Betta. There is a lot to this, from physical space to water quality. Fish release chemical signals, their form of "conversation," and other fish read these signals. All of this can cause stress, depending upon the species. Even if the neons (or whatever) never actually nipped the Betta, they are sending out signals that the Betta picks up, and this can unsettle him.
It can work in reverse too. The Betta, sensing this, can decide to take matters on his own, and snap at the neons. Bright colours tend to bring this out more, but it is still a risk.
And risk is something an aquarist must avoid. It is not fair to the fish to place them in surroundings that will cause them stress. It weakens them, causes health issues that wouldn't otherwise occur, and generally makes them "unhappy."
Betta are specifically bred to be fighters, and they need to be alone. It is cruel to the Betta to do otherwise.
Byron.
