To Mr.Pete Giwojna

Dear Dean:

Your 20-liter setup corresponds to a standard five-gallon aquarium here in the USA, and in this country a 20 L tank would be considered too small for any seahorses other than the dwarf variety (Hippocampus zosterae). The dwarf seahorses (H. zosterae) are pint-sized ponies that are no bigger than your thumbnail when they are fully grown. Since dwarf seahorses are native to Florida and the Gulf of Mexico, and the new CITES regulations protecting the entire genus Hippocampus now prohibit the importation/exportation of seahorses smaller than 10 cm in total length, it seems almost certain that your seahorses are not Hippocampus zosterae — the miniature marine marvels that are so popular with seahorse keepers here in America. And if they are any other species, then you’re 20 L tank may indeed be over stocked, sir.

Right now your seahorses are still pretty small, only about 2 inches (5 cm) in length, so they are probably fairly comfortable in your aquarium at the moment. But it seems very likely that they will quickly outgrow your small aquarium as they mature, and it would be very nice if you could upgrade to a larger aquarium for that reason. Do you know what species your seahorses actually are, Dean? And do you know if they are wild caught seahorses or domesticated seahorses that were born and raised in captivity? It would be very helpful in determining if your tank is seriously overcrowded if you could identify the species of pipefish and seahorses that you are keeping, but at least I can tell you that in general wild caught seahorses are much more vulnerable to crowded conditions than captive-bred-and-raised seahorses, so bear that in mind.

As for living ornaments or decorations for a seahorse tank, Dean, you can’t go wrong with colorful macroalgae and the macros will also help maintain good water quality in the aquarium by consuming nitrates and phosphates for growth.

For live hitching posts, I prefer decorative marine plants or macroalgae in a variety of shapes and colors and color — reds, golds, and yellows in addition to green varieties, some tall and feather, some short and bushy — to provide them with natural hitching posts and shelter. I like to start with a mixture of red and gold Gracilaria (Ogo) and artfully arrange them around a lush bed of assorted bright green Caulerpa. Any of the plumed (feathery) or long-bladed Caulerpa would be ideal for this, such as Caulerpa sertularioides, C. mexicana, C. ashmedii, C. serrulata or C. prolifera.

Be sure to thin out the fast-growing Caulerpa regularly; when you remove the excess fronds, you’re exporting phosphates, nitrates and other nutrients from the tank, thereby helping to maintain good water quality, and thinning out the runners helps keep it from going sexual.

When "pruning" or thinning out macroalgae, take care not to actually cut it. Remember, you’re not pruning hedges or trimming trees — the idea is to carefully pull up and remove continuous, unbroken fronds. Simply thin out the colony of excess strands, gently plucking up convenient fronds that can be readily removed intact. A little breakage is fine, but cutting or breaking too many strands will result in leaching undesirable substances into the aquarium water as the Caulerpa’s lifeblood drains away. Too much cutting or breaking can thus sap the colony’s strength and cause die offs or trigger the dreaded vegetative events that judiciously thinning out the colony otherwise prevents.

If you cannot obtain Caulerpa (it’s illegal in some coastal areas) or you’re simply concerned about your ability to maintain and control of Caulerpa properly, just use a different forms of macroalgae that grows less rapidly instead and you can get the same sort of benefits at relatively little risk. In that case, some of the other macroalge you may wish to consider are Gracilaria, Ulva, Chaetomorpha, and Chlorodesmis. Ogo (Gracilaria sp.) are bushy red-to-brown macros that do well under low light levels. Sea Lettuce (Ulva sp.) are deep green sheets of algae that do best under a little stronger lighting. Maiden’s Hair (Chlorodesmis sp.) are bright green tufts or clumps of very fine-bladed algal mats to grow attached to small rocks. All of these types of macroalgae are much less prolific and slower growing than Caulerpa. However, like all macroalgae, they should still be harvested periodically in order to export the excess nutrients they have consumed.

Aside from red and brown Gracilaria and the bright green Ulva and Maiden’s Hair, some seahorse keepers also like the Chaetomorpha turf algae or spaghetti algae, as it is also known. It can best be described as looking like the clumps of the colorful plastic grass we use to fill Easter baskets. Good on seven It is popular because it is slow growing and doesn’t require the kind of pruning that Caulerpa needs, and because it comes loaded with microfauna: miniature feather dusters, copepods and amphipods, tiny snails and micro stars. In short, Chaetomorpha is another interesting marine plant that can add some extra variety to a lush bed of macroalgae.

Many types of soft corals will also do well in a seahorse tank and can be considered as living ornaments for the right sort of setup. Soft corals have very little stinging ability and generally make good choices for a modified mini reef that will include seahorses. This includes mushroom corals (corallimorpharians) with the exception is the elephant ear mushroom anemone (Amplexidiscus fenestrafer), which is an active feeder on small fish.

You mentioned that you have an assortment of colorful live sponges in your 20 L aquarium, Dean. That’s all right providing you can keep the sponges healthy, and seahorses certainly do like to hang out amidst colorful sponges. However, live sponges are difficult to keep healthy in small, closed-system aquariums and typically require a large, well-established reef tank that can sustain filter feeding invertebrates in order to thrive. And you should be aware that many sponges contain toxic substances within their tissues, and they may release these toxins into the aquarium if they are injured. So treat your live sponges very delicately and carefully at all times. Don’t expose them to the air, make sure they don’t suffer any scrapes or cuts or fragmentation, and remove them from the aquarium promptly at the first indication that they are failing to thrive. It’s best to be cautious and play it safe when keeping live sponges.

Best of luck with your small seahorse setup, Dean!

Respectfully,
Pete Giwojna