Assuming that your aquarium will be a dedicated seahorse tank and not a community tank, and that you’ll be keeping captive-bred seahorses such as Mustangs or Sunbursts (Hippocampus erectus) of average size, the suggested stocking density under those circumstances is about one pair per 10 gallons of water volume. So a reasonable number of average size Mustangs (or Sunbursts) to keep in a 20-gallon aquarium is a total of about two pairs or four individuals. If you are new to seahorses, then I would recommend starting out with one pair of seahorses and then adding another pair after a few months, once you’ve had a chance to learn the ropes and gain a little viable firsthand experience keeping these amazing aquatic equines.
Of course, you’ll need to install a filtration system, cycle your aquarium, and decorate your tank before your Sea Ranch will be ready to support any seahorses at all. And when the big day finally arrives and you are finally ready to begin filling up the old corral with thoroughbreds, be sure to remember the three golden rules that should always guide your actions when stocking your seahorse setup:
I. Under stocking is ALWAYS better than over stocking. Always! That is the one immutable law that governs the seahorse-keeping universe, and if you violate it, the aquarium gods will exact swift and terrible retribution!
II. When in doubt, under stock. Don’t push your luck! If you have any doubt whatsoever as to whether or not your system is running at capacity, it probably is. In such a situation, you MUST err on the side of caution.
III. Don’t mess with success! If your seahorse setup has been running smoothly and trouble-free for a prolonged period at it’s present level of occupancy, try to resist the temptation to increase your herd. Why risk upsetting the balance in a system that has settled into a state of happy equilibrium? Rather than risk overcrowding an established tank, consider starting up a new aquarium when the urge to acquire some new specimens becomes overwhelming.
When stocking your aquarium, consider these golden rules to be your commandments. Obey them, and your system should flourish. Surely goodness and mercy shall follow thee all of thy days. Break them, and you will soon find yourself teetering on the brink of disaster. Abandon all hope ye whom embark down that dark road to ruin.
The height or length of a seahorse is measured from the top of its coronet to the tip of its extended tail, so those measurements are done when the tail is straight rather than curled.
What type of filtration are you planning on using for the 20-gallon hex tank?
Best of luck with your new seahorse setup!