Your water quality looks excellent — all of the aquarium parameters are right where they should be, so I don’t see any problem with your water chemistry and the biofilter seems to be handling the current bioload efficiently.
With regard to your cleanup crew, I would like to see you add a number of snails, particularly the omnivorous Nassarius snails to your sanitation engineers, since they will do a good job of cleaning up meaty leftovers such as any stray Mysis against by your Mustangs. I agree with Estefano regarding the sand sifting starfish, however — your sand sifter is problematic in a 24-gallon nano cube. They are difficult to keep alive and do best in large aquaria (55 gallons and up) with a deep live sand beds that have been well-established and therefore have built up a large population of meiofauna. There just isn’t going to be an enough fodder for a sand sifting starfish to thrive in a 24-gallon aquarium with a shallow bed of gravel and crushed shell, particularly a newly established tank. I wouldn’t consider adding a sand sifting starfish to any aquarium until it had been up and running for at least six months or a year. In your case, Yvonne, I wouldn’t get a sand sifter at all since they are not needed in an aquarium with a sand bed that’s just an inch or so deep, and your new seahorse system is really too small to support one. Perhaps your LFS will swap your sand sifting starfish for a couple of Nassarius snails instead.
The mushroom coral, star polyps, and button polyps are all good choices for a seahorse tank, and the pipe organ coral and brain coral should also be fine. You’ll have to be a little bit careful with the moon coral and trumpet coral, since they have large, fleshy polyps that can deliver a bit of a sting, but if you manage the circulation in the aquarium properly, they will probably work out all right in the long run as well.
You always have to be at least a bit cautious with large polyped stony (LPS) corals in a seahorse tank, Yvonne. LPS corals include genera such as Catalaphyllia, Cynarina, Euphyllia and Trachyphyllia that have large fleshy polyps which often have tentacles equipped with powerful stinging cells. The Euphyllia and Catalaphyllia have the most powerful nematocysts among the LPS corals, and can deliver stings that are stronger than most anemones (Delbeek, Oct. 2001). But the moon coral and trumpet coral you have are not that potent and will probably do okay with your seahorses as long as you observe the following precautions.
When designing a reef tank that will include seahorses, one must anticipate the different ways they might be injured in such a setup and then take precautions to prevent them from coming to harm. The process of rendering your reef system seahorse safe is much like the measures new parents take to childproof their house when they are expecting their first child. Intake tubes for the filters should be shielded, siphon tubes should be equipped with filter baskets or screens, and so on…
For instance, when powerful water movement is combined with overflows, there is a risk that seahorses could become pinned against an overflow or even go over it (Delbeek, Oct. 2001). Therefore, in the seahorse reef, overflows must be baffled and/or screened off, or the water flow should be adjusted sufficiently to prevent that from happening.
Likewise, although seahorses have no problem with strong currents in the wild, in the confines of aquarium, it is possible for them to come in contact with stinging corals if they are struck by a sudden powerful wave or surge, or are overwhelmed by a strong, unexpected current (Delbeek, Oct. 2001). The hobbyist needs to take this into consideration when placing water returns and corals in the seahorse reef (Delbeek, Oct. 2001). If possible, keep the water currents steady and unvarying so the seahorses can establish holding areas in the sheltered spots and low flow zones without getting blindsided by unpredictable currents.
Since your 24-gallon nano cube was designed with reef keepers in mind, it probably has fairly vigorous circulation, Yvonne. You’ll want to make sure that this doesn’t cause problems for your Mustangs at feeding time by swirling the frozen Mysis past them too fast for them to target and eat.
One indication that you may have too much water movement in your seahorse tank is if the seahorses are getting buffeted around by the currents, and whisked away uncontrollably when they tire of fighting the current. Or alternatively, they may stay perched in one place all the time and refuse to swim around and explore their tank for fear of getting swept away by the current if they relax their grip on their hitching posts. So you can get a pretty good gauge of how well the seahorses are able to cope with the water movement than their tank by observing how the current affects the swimming ability.
Likewise, if a mated pair of seahorses is consistently spilling eggs during the copulatory rise, that’s another pretty good indication that there may be too much turbulence or water movement in the upper reaches of their aquarium.
If the seahorses are having difficulty tracking their prey and eating because the current whisks the frozen Mysis past them too quickly to target it accurately and slurp it up, that’s another red flag. Often that situation can be corrected simply by adjusting the output from your filter to reduce the current during feeding time or turning it off altogether while a seahorses are eating.
But as long as your seahorses aren’t getting buffeted around, aren’t routinely dropping eggs during disrupted mating attempts, and aren’t having difficulty targeting their prey and eating, there’s really no such thing as too much water movement. In general, the stronger the water flow, the more important it is to keep the water currents steady and unvarying so the seahorses can establish holding areas in the sheltered spots and low-flow zones downcurrent without getting blindsided by unpredictable currents. Just make sure your seahorses are not getting trapped against overflows and be sure to screen off the intakes for any powerheads. Powerheads can be switched off at feeding time, if necessary.
Since the water chemistry in your aquarium looks very good at the present stocking density, I think it’s all right to consider adding a couple of peppermint shrimp and a small, passive fish such as a clown gobie, providing you upgrade the filtration system as we have previously discussed and quarantine any new fish before you introduce them to the seahorse tank.
However, Yvonne, rather than a clown goby, you may want to consider a Six Line Wrasse (Psuedocheilinus hexataenia) instead. The wrasse is also a reef-safe fish that will make an excellent tankmates for your seahorses, and it has the added virtue of eating small bristleworms, so adding a six line wrasse may help you to keep the bristleworm population in check.
Best of luck with your new seahorse tank, Yvonne!