I have three clowns (4"-5") in a 260 gallon tank with my 9" Male Marbled Motoro and have never had any issues. Actually, quite the opposite, as the clowns do a great job cleaning-up the scraps of market shrimp that I feed the ray. I have even seen the three clowns swimming on-top of the stingray as it moves through the tank...pretty cool to watch them swim together accross the tank.