Hi, all. So I decided last week to try seven neon tetras in with my male betta in his 12g Edge. Advice, when I sought it, was incredibly mixed, but most of it came down to the fact that some bettas would be fine with this and some would not. I decided that close watchfulness for several days would be worth the risk. I tried one tetra, then three, and he was utterly uninterested after a full day. I added the remaining four.
FIRST WARNING: Watchfulness is not sufficient.
Despite looking at the fish often, I never once saw my betta chase a tetra, though I did see the tetras fighting among themselves. However, several days later, I couldn't find two of the tetras. I do have some rocks that they can hide in, and I decided they must be there. I didn't see any bodies.
SECOND WARNING: If any tetras go missing, immediately remove remaining tetras.
When I woke up the next morning, I was down to no tetras, and my betta was consuming a dead one at the bottom of the tank. Out of the seven, I only found two bodies. My assumption is that the betta ate the rest, because he now is bloated to to the point of dropsy. I have no idea if he will recover.
THIRD WARNING: If you decide to pursue putting tetras in with your betta, and your betta is one who simply needs to be alone, you are endangering not just the tetras but your betta as well.