As a member of the piranha family, tetras are far too nippy to be kept with bettas, in my honest opinion, especially my guy who is like 90% fin. I know they're a popular freshwater fish, especially with bettas, so I added them to the list.
Well, you've got it a little backwards - piranhas are in the tetra family.
By and large, tetras are not that nippy in my experience. I have kept some species that were, but most are not. Too, nippiness has a lot to do with circumstances. Your tank would be plenty large to keep proper schools, and for fish to have space.
When I had my 30, the betta was in there with black neons, orange flame and regular neon tetras.
You are certainly welcome to not want to "risk" it, but I don't think it's right to say that tetras are far too nippy just because they are related to piranhas, or because a few species happen to be. The same thing happens with barbs - because tiger barbs are aggressive, people will say that all barbs are aggressive, and nothing could be farther from the truth. Or because certain dog breeds "are aggressive" therefore all breeds are aggressive. Generally such blanket statements are not that accurate.
Bettas can eat platy fry, but not the platys themselves.
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125 - BGK, chanchito cichlid, pictus cats, silver dollars, palmas bichir
125 - cichlids (severums, bolivian rams, chocolate), rainbows ( turquoise, red), loaches (angelicus, zebra, kuhli and horseface), plecos (BN, RL and clown), denison barbs, tiretrack eel, pearl gouramis, betta
90 - Congo tetras, african knife, upside down cats, spotted ctenopoma, kribensis, delhezzi bichir
2.5 - betta