I prefer to use pre-cured, "debugged" live rock to provide all the biofiltration for my seahorse tanks with a simple external filter to provide water movement along with mechanical and chemical filtration along with a good protein skimmer to improve water quality.
I recommend 1-2 pounds of live rock per gallon if it’s going to serve as the primary means of biofiltration. That amount of live rock will provide all of the nitrification AND denitrification the aquarium requires in order to carry out the complete nitrogen cycle and keep your nitrates nice and low. I like to select choice pieces of live rock heavily overgrown with pink and purple coralline algae for the top layer of live rock.
A thin layer of live sand, preferably black, is the ideal substrate for a SHOWLR 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 shows off my seahorses and macroalgae to great effect and enhances the appearance of tank in general.
An external power filter is a valuable addition to any seahorse setup for several reasons. It will provide added water movement and circulation for your aquarium, as well as accommodating any mechanical or chemical filtration you may desire. A bewildering array of filtration options are available today, including a myriad of canisters and hang-on-the-back models, most of which will do the job reasonably well. Even the trusty old standbys, undergravel filters and air-operated sponge/foam filters, are still good choices for a standard seahorse setup.
If you are using 1-2 pounds of live rock per gallon as your primary biofilter, then a basic canister filter or hang-on-the-back filter is all you need to provide mechanical/chemical filtration and additional water movement. Otherwise, you’ll need to include a filter that can provide biological filtration. Undergravel filters and air-operated sponge filters can accomplish this and work well for some applications.
But wet/dry trickle filters are probably the most desirable units for the seahorse keeper if the aquarium has adequate space (behind or beneath it) to accommodate such a unit and the hobbyist can afford one. They are top-of-the-line units that feature a thin film of water trickling over filter media with an ultra-large surface area, thereby allowing maximum air-water contact. This provides excellent oxygenation with efficient offgassing, which is very important for seahorses. It helps keep dissolved oxygen levels high, CO2 low, and effectively prevents gas supersaturation, which can sometimes contribute to serious problems for our aquatic equines. As a added benefit, wet/dry trickle filters can also provide remarkable biological filtration, which can give you a real nice edge by further increasing your carrying capacity and boosting your margin for error accordingly. Biowheel filters provide many of the same benefits as wet/dry trickle filters and are also a good choice for the seahorse keeper.
The type or brand of supplemental filter you choose for your seahorse tank is not critical, but there are certain desirable features you want to look for in any filter that will be used for seahorses. For example, it should provide good surface agitation and water movement with adjustable flow. The intake tubes should reach all the way to the substrate (add extenders if they do not) and be screened off or otherwise shielded so they cannot "eat" a curious seahorse. The filter must provide efficient oxygenation and gas exchange and be able to accommodate mechanical and chemical filtration media such as activated carbon and polyfilter pads. A prefilter is very desirable, as is a "waterfall" return (or a spray bar attachment, if you decide on a good canister filter instead).
Good luck finding just the right filter for your needs and interests, Captain!