I have always been under the impression that the biggest problem with MH over seahorse tanks is that they create alot of heat, making it more difficult to keep the tank at the lower temps most seahorses require (at least lower than most people keep their reef tanks). I would think that as long as some shady areas were provided and you deal with the heat issues, seahorses should be fine under MH, given their natural habitat....
Just my $0.02
