I would caution you to watch the temperature of the tank.
Being covered over like that may possibly lead to elevated temperatures that can be problematic for seahorses in captivity.
In their natural habitat, bacteria are not contained and ones like the vibriosis don't become a problem. In our tanks though, vibriosis and some others are in a captive, nutrient laden tank that make their numbers increase exponentially with each rising degree, especially above 74°F. Some succeed not too bad up to 78° but it means excessive husbandry, especially when you have to keep the tank cleaner than a reef tank even at lower temperatures.
While most of our other salt fish are able to deal with this, seahorses don't have the same ability.
Dan Underwood of seahorsesource.com explains it best in "A Commentary by Dan Underwood" which is linked to at the bottom of
My Thoughts.... page which is also linked in my signature.
As a side note, the only TRUE captive bred seahorses coming into Canada at this time are reidi and comes from Aquamarine International, and dwarfs and erectus coming from Seahorse Corral in Florida.
The remainder are "tank raised" or "net pen raised" in ocean water that has not been sufficiently treated and filtered for pathogens as the ones for the hobby are just a small percentage filtered off from the millions that are being bred for the Asian medicine trade.
True captive bred seahorses are raised in commercial salt water like I.O. or in ocean water that is sufficiently treated to remove the pathogens.