I'm awaiting my dwarf seahorse order, held up due to cold weather now after many other delays, but from the information I've got from seahorse.org, and from my experiences of 17yrs of reefing, 5 of seahorse keeping, you are better off with the top open for better gas exchange and keeping the pH up.
Some use power filters, protected so the seahorses can't get sucked up, but a lot use the air powered water motion, some with sponge and some without.
I personally don't like the sponge because it is a "first" source of bacteria contamination if you forget to clean it out very frequently and seahorses of any kind are extremely susceptible to bacterial infestations.
Nightly top up to replace evaporated water is best as you need to keep the salinity from having big swings.
Dry rock works well as it doesn't introduce things like hydroids, it just needs ammonia source added to cycle the system.
Water changes vary with the keeper and the particular system and contents.
I have some seahorse tanks that need changing every 2 to 3 weeks and others can go for a month.
The live bbs should be ongrown for 24 hours after hatchout and then enriched with a product like Dan's Food for two 12 hour stages and then placing enough in the tank to give sufficient density so the dwarfs can feed on the food as it passes by while they are still hitched. Many dwarfs don't like to hunt down their food so denser feeding is required.
You need to turn off any filter/powerhead while feeding, but also, remove the excess food after the feeding period before you turn the filter/powerhead back on.
