Sounds like the best thing to do is get rid of the fish before you become any more attached to it. No, it cannot be kept in anything near as small as the tanks you have. Not for long at least. While it is small now, they get massive, ultimately requiring a fish pond. Assuming that you provide it the care that it needs. It does not sound like you are in a position to provide that care, so again the best thing to do is to give it up.
It will eat your tetras the second it's large enough. Possibly a little before. It will be large enough to eat them in about 6 months or so. Again, assuming proper care.
Last edited by jaysee; 10-13-2013 at 12:30 AM.