I would be happy to help you get your new 20-gallon aquarium up and running, and to discuss the different specimens that you are considering for this setup.
For starters, I have found that the royal gramma (Gramma loretto) does great with seahorses, birdle. Royal grammas are highly territorial and very quarrelsome amongst themselves, but for all practical intents and purposes, it’s been my experience that they utterly ignore seahorses (and vice versa). They have brilliant colors, a docile disposition towards seahorses, and are deliberate feeders that won’t outcompete the ponies at mealtime. As long as you are willing to limit yourself to just one Royal Gramma and quarantine it before you introduce it to the main tank, I’m quite confident it will make a wonderful addition to your seahorse tank.
Bonus Tip: if you do decide to try a royal gramma, see if you can obtain a Gro-Lux fluorescent tube to use in your light fixture. Osram Gro-lux bulbs put out wavelengths of light that are concentrated toward the red and violet regions of the spectrum. They are intended to stimulate better plant growth, but have the added affect of greatly enhancing any red or orange or purple colors they illuminate. When bathed in Gro-lux light, bright red or orange seahorses literally glow! And so do Royal Grammas — the magenta coloration of these fishes will all but fluoresce under Grolux lighting. The pinkish-purple end of these bicolor beauties will be instantly suffused with a dazzling hot-pinkish purple glow that the ends abruptly where there yellow half begins. Dazzling!
The purple firefish goby (Nemateleotris decora) and dartfish or firefish goby (Nemateleotris magnifica) will likewise make great take makes for seahorses. They are also very colorful, peaceful little fish that are shy deliberate feeders.
But you will need to reconsider your thinking with regard to the Mandarin Dragonet (Pterosynchiropus splendidus) and the Scooter blenny (Synchiropus sp.), which is actually not a blenny at all but another type of dragonet. Although Mandarin fish and scooter blennies are typically wonderful tankmates your seahorses, a 20-gallon aquarium is simply not large enough to sustain them because of their dietary requirements.
Don’t get me wrong, birdle — I absolutely love the psychedelic coloration and peaceful nature of Mandarin dragonets! There’s no disputing that they are gorgeous little fishes and make ideal tankmates for seahorses in the right type of setup. They are docile, slow-moving, passive fish that are beautifully marked and very deliberate feeders. And they are quite hardy fish providing they can be fed properly. They have a heavy slime coat that seems to make them quite resistant to protozoan parasites such as Cryptocaryon irritans.
But, as you know, in order to do well, mandarins need a large, well-established aquarium loaded with live rock or live rock rubble that’s teeming with copepods and amphipods. Mandrins must have continuous opportunities to graze on suitable live foods or they generally slowly waste away and starve to death. In the right system, they can thrive, and will often learn to take small pieces of frozen Mysis, but they do best in well-established reef systems or aquariums with at least 1 pound of live rock or LR rubble per gallon, a mature sand bed, and a refugium that can continually replenish the pod population in the tank. Those are typically the conditions that are necessary to assure they have adequate suitable live prey.
Mandarins are bottom feeders that normally do not take food from the water column, so select an aquarium with a large foot print that can accommodate plenty of live sand, small pieces of live rock, live rock rubble, and macroalgae. In other words, this generally means a well-established reef tank of 100 gallons or more.
When discussing compatible tankmates for seahorses, it’s important to remember that one can only speak in generalities. There are no unbreakable rules, no sure things, no absolute guarantees. For instance, most hobbyists will tell you that small scooter blennies make great tankmates for seahorses and 9 times out of 10 they’re right. But every once in a while, you will hear horror stories from hobbyists about how their scooter blenny coexisted peacefully with their seahorses for several months and then suddenly went "rouge" overnight for no apparent reason and turned on the seahorses, inflicting serious damage before it could be captured and removed.
Does that mean that we should cross scooter blennies off our list of compatible tankmates for seahorses? Nope — it just means that we must be aware that individuals within a species sometimes vary in their behavior and respond differently than you would expect, so there are exceptions to every rule. It’s fair to say that scooter blennies generally make wonderful companions for seahorses, but there’s always a small chance you might get Satan reincarnated in the form of a scooter blenny. There’s no guarantee that adorable scooter you picked out at your LFS because of his amusing antics and puppy-dog personality won’t turn out to be the blenny from hell once you release him in your seahorse setup.
In short, although scooter blenny would most likely make a fine tankmate for seahorses, scooters are pod eaters just like Mandarin dragonets, and this means they also require large, well-established aquarium’s with live sand and lots of live rock in order to thrive. They are another good candidate for large reef tanks and excess of 100 gallons, but a Mandarin fish or a scooter blenny would be doomed to a long slow death from starvation in a 20-gallon aquarium.
The peppermint shrimp get a big thumbs up, birdle! Peppermint shrimp (Lysmata wurdemanni) are a favorite with seahorse keepers because they eat Aiptasia rock anemones, and both the peppermints and Scarlet cleaner shrimp (Lysmata amboinensis) will perform another useful service by grooming the seahorses and cleaning them of ectoparasites. As an added bonus, they reproduce regularly in the aquarium, producing swarms of larval nauplii that the seahorses love to eat.
Just remember, it is important to select good sized cleaner shrimp for your seahorse tank(s). Seahorses will actively hunt small cleaner shrimp and they are quite capable of killing shrimp that are far too big to swallow whole, so the cleaners need to be large enough that they are not regarded as potential prey.
Another thing to keep in mind when introducing cleaner shrimp to your aquarium is that they are more sensitive to water quality and rapid changes in pH, temperature, or salinity than fishes are, meaning the shrimp need to be acclimated more carefully and gradually. Whereas drip acclimation should be avoided for seahorses that have been on the shipping bag for 24 hours or more, it is the perfect way to acclimate delicate shrimp from your LFS. They will do best it drip acclimated to the new aquarium over a period of several hours to allow them to adjust to any differences in the water parameters very gradually.
Shrimp that are introduced to a new aquarium too abruptly will not flourish and are liable to die within a day or two from the stress of acclimation, unable to adjust to any significant differences in pH or salinity, or they simply fail to thrive and expire a week or two later for no apparent reason. If the shock is too great, they will autotomize, dropping legs, claws and/or antennae immediately upon being introduced to the new aquarium conditions.
As far as starfish go, it’s best to avoid a large predatory species such as chocolate chip starfish and African red knob starfish (Protoreaster spp.). I would describe predatory sea stars such as these as "opportunistic omnivores," meaning that they are likely to eat any sessile or slow-moving animals that they can catch or overpower. For instance, I would not trust them with snails, clams, tunicates, soft corals and the like. Most fishes are far too fast and agile to be threatened by sea stars, but seahorses are sometimes an exception due to their sedentary lifestyle and habit of perching in one place for extended periods of time. What occasionally happens, in the confines of the aquarium, is that a predatory starfish may pin down the tail of a seahorse that was perched to the piece of coral or rock the starfish was climbing on, evert it’s stomach, and begin to digest that portion of the seahorse’s tail that is pinned beneath its body. That’s a real risk with large predatory species such as the beautiful Protoreaster starfish are the popular chocolate chip stars, which are surprisingly voracious and aggressive for an echinoderms.
But there are a number of colorful starfish that do well with seahorses. Any of the brightly colored Fromia or Linkia species would make good tankmates for seahorses. However, bear in mind that, like all echinoderms, sea stars are very sensitive to water quality and generally will not do well in a newly established aquarium. Wait until your seahorse tank is well-established and has had a chance to mature and stabilize before you try any starfish.
Three attractive species I can recommend are the Fromia Sea Star or Marbled Sea Star (Fromia monilis), the Red Bali Starfish (Fromia milliporella), and the Red Starfish (Fromia elegans), which are safe to keep with seahorses. They are not nearly as delicate as the Linkia species and should do well in the tank such as you’re planning that has lots of live rock and optimum water quality, and are nonaggressive starfish that feed primarily on detritus and meiofauna on live rock and sandy substrates.
Leather corals of all kinds and pulsing Xenia are among the soft corals that do exceptionally well with seahorses, so you may certainly include a finger leather coral, colony of pulsing Xenia.
In short, birdle, all things considered, I would eliminate the Mandarin fish and scooter blenny and alter the list of specimens on your wish list as follows:
Royal Gramma (Gramma loretto)
Purple firefish goby (Nemateleotris decora)
Dartfish or firefish goby (Nemateleotris magnifica)
Colorful Fromia starfish (rather than a chocolate chip starfish)
Peppermint Shrimp (Lysmata wurdemanni)
Finger leather coral
Those are all specimens that would be compatible with seahorses and that you may be able to keep in a well-filtered 20-gallon aquarium when the biological filtration is fully mature and the aquarium is fully stocked. I would recommend adding some live rock to the aquarium to provide stability and additional biological filtration.
If you are serious about keeping seahorses, birdle, then I would also encourage you to participate in the Ocean Rider training program for new seahorse keepers. You can read all about the seahorse training program in the very first post at the top of this discussion forum, but the bottom line is that it’s a correspondence course conducted entirely by e-mail and it’s entirely free of charge. It will tell you everything you need to know about the care and keeping of seahorses in great detail, and completing the training course should give you an excellent idea of whether are not they are well suited for your needs and interests. If you would like to give the seahorse training program a try, just send me a brief e-mail off list ([email protected]) with your full name (first and last) and I will get you started off with the first lesson right away.
One of the lessons in the seahorse training course is devoted to a detailed discussion of compatible tank mates for seahorses, including compatible fish, invertebrates, and live corals. And the very first lesson in the training program explains how to optimize your aquarium to create ideal conditions for seahorses, and discusses all of the different pieces of equipment and aquarium accessories you will need in detail. In other words, the seahorse training program is tailor-made for hobbyists like yourself that are in the planning stages of setting up a new aquarium for seahorses.
Best wishes with all your fishes, birdle!