It will still trap detritus, even the finest of stuff, and will eventually cause you problems. Empty it and use it as circulation only, forget that it's intended to filter the water and pretend it's a pump. Trust me, and I'm sure someone else on here will agree with me.

The live rock will do you better in your tank, where it can both nitrify and denitrify. Forcing water through the rock like that will turn it into a biological filter, effectively eliminating it's ability to perform denitrification inside the rock.

Seahorse tanks do work a little differently then reef tanks, especially if you have lots of macro algae where you actually want a slight accumulation of nutrients to feed the algae. Plus in general the bioload on the tank is kept to an absolute minimum.

I suggest you visit www.seahorse.org, they have a wealth of information on seahorse setups, and they helped me out when first setting up my seahorses. Consider the natural environments of seahorses in the wild: seagrass beds and gorgonian fields, and realize that these areas are often full of nutrients and detritus. Obviously waste accumulates very quickly in aquaria thus it needs to be monitored, but a little waste buildup in a filter should be nothing to panick about as long as it is maintained monthly. I would not reccommend it for a reef tank, but for FO or FOWLR it seems to work out fine. :)

-Diana