If you treat them as brackish, like sailfin mollies they should top out between 18-20" a little bigger full salt, no matter how you treat them they only live 5-6 years. At 18" they are still not much than an inch thick and while they will suck up baby guppies and glass shrimp they can't be considered predatious. I have kept a couple and always traded them off when I converted the tanks to predators. Most small cats and other fish love tubis, the problem is they got a bad rep from a few suppliers, black worms are the modern equivalent and in a lot of cases just as dirty at twice the cost. I used to wild catch my own.