lol..I know what plenums are..and personally Ive never seen anyone be able to maintain a plenum successfully and definately wouldnt recommend it to beginners. Besides that though..plenums are not recommended for elasmobranch's.

1. The biggest concern being that sharks and rays often disturb the substrate which would release all the gunk in a plenum into your tank.

2. With these kind of animals biological filtration is severely inadequate. Live subtrate is OK for a small shark or ray..but they dont stay small. Heavy 'outside' the tank filtration is needed. IE: pressurized canister systems, wet/dry's, fluidized bed filter etc.

3. sharks and rays are easy to overfeed and have a propensity to regurgitate..rapidly increasing the waste in a tank and most 'live subtrate' tanks wont have adequate nitrifying bacteria to break down the sudden 'extras.'

If you have a plenum ...great...I dont think its wise...but different methods work for different people. However, the care of the animals comes first, and plenums or DSBs are not the best solution for a tank with animals that will most likely send the substrate flying and release all that 'gunk' into your tank...not a healthy situation.