The cowfish reaches a full grown size of 1'8" according to liveaquaria. Also it is difficult to care for and can poison your tank if it gets stressed. Cool fish, wouldn't want him to suffer in that small tank. The valentini suffer should be ok. General rule of thumb to follow, one inch of fish for every five gallons. That puffer full grown will be 4 inches, so becareful not to overstock that small tank.