you can keep 1 but i would suggest waiting for petcos dollar per gallon sale and get a 30 long and house 2 because even though they can get aggressive to there own kind, they do better with another of their own. the aggression only happens during feeding. right now i have 2 young mudskippers in a temporary 10 gallon and they get along fine. they are always next to each other the only time i see them get aggressive is when i feed them and its never that bad. Im waiting for a sale at petco or a good deal on craigslist to upgrade their tank.