Shorty and Leslie are both correct — a well-filtered 20-gallon aquarium with sufficient height could support two pairs of Sunbursts if you practice good aquarium management and keep up with an accelerated maintenance schedule, but that’s not something that I would recommend for anyone who is new to seahorses.
The suggested stocking density for Mustangs and Sunbursts (Hippocampus erectus) is one pair per 10 gallons of water volume. Of course, that does not mean that you can keep H. erectus and a 10-gallon aquarium; rather, those are just general guidelines that indicate if you have a properly maintained 30-gallon aquarium, for example, it is spacious enough for up to three pairs or six individual Mustangs or Sunbursts. So a well-filtered, well-maintained 20-gallon aquarium could theoretically support two pairs of Sunbursts, but an inexperienced seahorse keeper should not attempt to keep his tank stocked to capacity.
If you’re a rank beginner, you will be better off keeping your stable under stocked in order to provide a margin of error while you learn the ropes with these amazing aquatic equines. Savvy seahorse pros who’ve seen it all before and know all the tricks and trouble spots, on the other hand, can afford to push the envelope a bit and keep their herds near capacity (Giwojna, Jan. 2002). In other words, an experienced seahorse keeper could maintain two pairs of Mustangs or Sunbursts (Hippocampus erectus) without problems, but that would not be advisable for your first serious attempt at keeping seahorses.
As always, be sure to remember the three golden rules that should always guide your actions when stocking any 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.
Best wishes with all your fishes, Nigel!