Leslie is correct — a 12 or 16-gallon aquarium is really too small for any of the greater seahorses such as Mustangs or Sunbursts (hippocampus erectus). Such small tanks lack sufficient height or bottom space for the larger breeds of seahorses, and have inadequate water volume to provide stable water conditions.
Unless you will be keeping one of the miniature breeds of farm-raised seahorses, such as Hippocampus zosterae, H. breviceps, or H. tuberculatus, it’s generally best to start with the largest aquarium you can reasonably afford and maintain (the taller, the better). In general, a tank of at least 40 gallons (150 L) is preferable since that’s the size when one begins to see significant benefits in terms of the greater stability a larger volume of water can provide. An aquarium of 40-gallons or more will be more resistant to overcrowding and to rapid fluctuations in temperature, pH, and salinity than smaller setups. The larger the aquarium the larger the margin for error it offers the aquarist and the greater the benefits it provides in terms of stability.
It is equally desirable to select an aquarium at least 20-inches high when keeping the greater seahorses. They need the vertical swimming space to perform their complex mating ritual and successfully complete the egg transfer, which is accomplished while the pair is rising through the water column or drifting slowly downwards from the apex of their rise. If the aquarium is too shallow, eggs will be spilled during the transfer from the female to the male’s brood pouch, and mating becomes increasingly difficult or impossible below a certain minimum depth. A tall aquarium can also help protect the seahorses from depth-related health problems such as bloated pouch and certain forms of Gas Bubble Disease.
Zulu-lulus (Hippocampus capensis) are medium-size seahorses. They top out at around 4 inches long maximum, with most adults being around 2-3 inches in height. Like Leslie, I prefer to see them kept in aquaria of around 20-30 gallons for best results. But if you are willing to forego the Mustangs, and limit yourself to know more than a trio or perhaps a quartet of the Zulus at most, a small group of them might do well in a well-planted species-only tank of 16-gallons with good filtration. Let me know if you are willing to consider limiting yourself to a few Zulus in such a tank, and I will happy to provide you with some suggestions on how best to set it up. You would need to be able to maintain stable temperatures in the 72°F to 75°F range in order for the Zulu-lulus to do well.
Best wishes with all of your fishes!