What to do about nipping in tank?

Return the Tetras.

RN Tetras need 20+ gallons. Cramped fish will be aggressive/nippy fish.

In lieu of the Tetras, look for fish more compatible to your Betta and aquarium volume.

Some you might consider include:

Pygmy Corys. Celestial Pearl Danios. Dwarf Rasboras.

Good luck.

FYI, good fish profiles that include minimum aquarium size, grouping needs, and compatibility:

By species/kind: http://www.peteducation.com/category.cfm?c=16+1911

Alphabetically: http://badmanstropicalfish.com/profile_alpha.html

Search-by-name: http://www.aquariumlife.net/fishprof.asp

The tetras should really have at least a 20 gallons, a lot of tetras can be fin nippy. Especially when crowded or stressed. You wont really be able to stop this, you need to seperate them asap. And add some salt and or a stresscoat product to the betta tank to prevent infection and promote healing.

Sounds like your tetras have been in a single species breeding tank their whole life where they picked up such bad habits. You'll want to separate them and try breeding the tetras so you can grow the fry in a community tank where they will learn to be peaceful.

Introducing a third species to the tank could help as well, perhaps some zebra danios which are typically good socializers.

First of all...Bettas need air as you should know because they dont breath like normal fish. Also keeping your betta with other fish is usually not a very good idea because they dont like currents especially if they are males. I highly suggest getting a betta bowl because they dont like being with other colorful fish.

As for the nipping no it will only continue because most fish have a tendency to nip at colorful fins especially guppies. If you consider you probably feed them fish flakes that look rather colorful...your bettas tail looks the exact same except it MOVES!

Overall...I would advise moving your betta out before its too late.