Yes, sir, I use sand in all my seahorse setups and I have never had any problems. I find that a fine-grained oolitic live sand, preferably black, helps control nitrates and looks great, as discussed below. (The CaribSea Indo-Pacific black extra-fine sand should fit the bill nicely.)
A thin layer of live sand is the ideal substrate for a seahorse tank. It is bioactive, aesthetically pleasing, and is a fine-grained sand well suited for the various snails that form an essential part of the cleanup crew for a seahorse tank. I find the dark color of an oolitic black sand substrate shows off my seahorses and macroalgae to great effect and enhances the appearance of tank in general.
As long as it’s fine enough I’ve never had any problems with seahorses "snicking" up sand in the aquarium. They will do so on occasion when feeding off the bottom, but never have any difficulty at all expelling it again as long as it’s fine grained. So I find that very fine "sugar" sand works for a well for seahorses, and you don’t have to worry about any pebbles or bits of rock that are too large to be accidentally ingested. What is sometimes problematic are small pebbles or bits of rock that might be accidentally slurped up a seahorse’s tube snout when they are feeding on the bottom, and which could then be difficult for the seahorse to expel again. So if you want to shift through the Indo-Pacific black sand to leave only the fine-grained oolite, that may be a sensible precaution.
The depth of a shallow sand bed like this is also an important factor. Too deep, and you risk anaerobic dead spots where deadly hydrogen sulfide gas can form. Too shallow, and there will be less surface area to support beneficial nitrifying bacteria and Nassarius snails and other beneficial burrowers may feel vulnerable and exposed. A bed of live sand between 1/2 to nomore than 1-inch deep is just right for the main tank. A properly layered Deep Live Sand Bed (DLSB) 3-6 inches deep with a full complement of sand shifters also works well with seahorses, but is best confined to a sump rather than the display tank due to the seahorse’s heavy waste production. In other words, you can minimize the buildup of detritus in the DLSB by installing it in your sump rather than the main tank.
The types of live sand I prefer are either the Arag-Alive Indo-Pacific Black Sand by CaribSea or else the CaribSea Tropical Isle Tahitian Moon Black Sand. You can obtain them online from Premium Aquatics and a number of other sources, and either of them should work well for the substrate in a seahorse tank.
Best of luck with your new seahorse setup, shatteredstars!