I think it’s a fine idea to increase the biofiltration in your seahorse tank, but I would be inclined to do that by adding more live rock rather than live sand. Seahorses are such messy feeders that I prefer only a thin layer of live sand in the main tank (a properly maintained DLSB can do wonders, particularly for controlling nitrate levels, but is best confined to the sump on a system that holds seahorses).
Live rock will provide both additional nitrification and denitrification ability, so by all means, feel free to add several more pieces of attractive, choice live rock if you wish. But I would stick with just a thin layer of live sand, as discussed below:
A thin layer of live sand is the ideal substrate for a seahorse-only-white-live-rock 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.)
The depth of a shallow sand bed like this is a crucial 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 know more 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.
As for the new seahorses you are expecting, I would hold off on adding any new specimens until you’re sure that your water quality is back to optimum and your tank is in tip-top condition. Be sure to look over the disease prevention and control measures and suggestions for rehabbing your aquarium after an outbreak of disease in the following discussion thread:
Click here: OceanRider : Message: Re: Sick seahorse-please help! <http://groups.yahoo.com/group/OceanRider/message/9573>
Once you’ve made any adjustments to your aquarium that seem advisable and you’re sure all is well, then you can go ahead and reschedule the delivery of your new Mustangs.
Best of luck with your seahorse setup, Kris!