I have a well established 15 US Gal. Long ... 2 cubic sq.ft in volume. Heavily planted. In it live adult fish consisting of (8) Neon Tetras adult 1.5" (8) H. Rasboras 1.5" (3) Corydoras Trillineatus 2.5" (x>6) adult ghost shrimp 1-2.5"+ ?baby shrimp. (1) mystery Snail 6" when fully out... Almost impossible to take a ghost shrimp census, but I have babies becoming adults some-how. As in JURASSIC PARK, "Life found a way."
The betta I put in this tank is short of being a fry. The tank is heavily planted, with a diverse bio-dome. Is this cruel? The betta seems to be happy hiding in the plant cover close to the surface. I estimate he is .5 inches in length, making my smallest neon tetra 3X his size at the least.
I feed my fish live brine shrimp regularly. With such a diverse group of animals; all of which are omnivores; there should be no fear of starvation. The betta will grow up being the small-fish in the pond, and hopefully in theory as an adult (when he is the big fish in the pond) he will be nice and courteous to his mates.... That is the theory. I do have a 1 gallon acrylic tank, on stand by; if dumping a small guy like this is deemed too much.