Many saltwater aquarists, once they have been in the hobby for a while, look for something a bit different and more interesting in marine aquaria. To a growing number of people looking for a new challenge, successfully keeping seahorses in a saltwater aquarium fits the bill perfectly.
Up until fairly recently, seahorses have been considered to be difficult to impossible to keep alive in an aquarium for any length of time. The first attempts were made using wild caught seahorses. These specimens usually perished in a few months either from starvation or infections. Once the seahorse keeping pioneers determined how to breed them in captivity (aquaculturing) the major obstacles were soon overcome and keeping seahorses in an aquarium long term became doable. Unlike wild caught seahorses, captive bred seahorses will readily take hand-fed foods.
Among the many fishes in the ocean, seahorses are some of the strangest looking. They have an exoskeletal (like a crustacean) body covered with a type of skin rather than scales. This makes the seahorse more skeptical to external injuries and infections than most fish. A seahorse's gills are less well developed than most bony fish and are therefore less efficient at exchanging gas. This must be kept in mind when designing an aquarium for a seahorse.
There are over 30 recognized Hippocampus genus member seahorses that inhabit tropical, sub-tropical and temperate waters around the world, but only a handful of species are typically encountered in the aquarium trade. From the few wild and numerous tank-bred specimens one finds for sale, the Smooth Seahorse (Hippocampus kuda) and the Lined Seahorse (Hippocampus erectus) are at the top of the list of the most common species of seahorses that are seen in aquariums. Also becoming popular is the Dwarf Seahorse (Hippocampus zostera) which, in the wild, is found in the western Atlantic Ocean, from Bermuda to the Bahamas, southern Florida and the entire Gulf of Mexico. As the name Dwarf Seahorse implies, this is a tiny species that attains less than two inches in size, which makes them ideal for mini or nano-reef tanks.
A seahorse tank should be at least 18 inches tall. Seahorses are more vertically oriented than horizontally. That is to say that they like to move up and down in an aquarium more than side to side. The DIY Seahorse Tank that we designed for two Dwarf Seahorses had a 12" by 12" footprint and was 22 inches tall. This size worked well for two Dwarfs the Striped Mandarinfish we kept in the tank, but if you are going to keep more or the larger seahorse species, you will want a tank with a larger footprint.
Because they are not strong swimmers (and because their gills are not very efficient), seahorses do not like strong water movement. For this reason, we used a hang-on-the-back (HOB) Power Filter on our DIY Seahorse Tank. Even though the tank was only about 12 gallons in size, we used a power filter that was rated for a 20-gallon tank. We also extended the pickup tube to reach almost all the way to the bottom of the tank. This provided much better vertical water flow in the tank, improving the gas exchange at the water surface. The extended tube also helped to keep the bottom of the tank free of Seahorse manure. Seahorses have a short and very primitive digestion tract, which allows a great deal of undigested food to end up on the bottom of the tank, adding to water quality issues.
Seahorses need at least one hold fast or hitching post in the tank so they don't have to be constantly swimming. If they aren't looking for food, seahorses will spend most of their time with their tails wrapped around almost anything that will hold them in place. Gorgonians work great for hitching posts, as do faux corals or even plastic aquarium plants.
Seahorse Food & Feeding
Seahorses should be fed at least twice per day. The best food for captive bred seahorses is frozen Mysis shrimp. If you obtain an aquacultured seahorse, it is probably already eating Mysis, so getting it to eat in your tank should not be a problem.
While you can just squirt some thawed Mysis into the tank and let the seahorses chase it around until they catch some, target feeding your ponies in a low water flow area of the tank will make it easier for them to get enough food without having to work too hard. We used to turn off the filter on our tank and then target food them while they were hitched to their post. When they had eaten their fill, we turned the filter back on.
Seahorse Compatible Tankmates
Seahorses are not aggressive feeders so any fish or invertebrates you include in the tank should be slow, cautious eaters. We found that the Mandarinfish we kept with our Dwarfs worked perfectly. Ironically, the Mandarin was harder to train to eat Mysis than the seahorses were. We had the Mandarin eating out of the tip of an eyedropper, then she would chase the Mysis around the bottom of the tank until it caught them.