When it comes to aquarium height, the taller the better is the rule for seahorse keepers. This is because seahorses need vertical swimming space in order to mate comfortably and carry out the exchange of eggs, but more importantly, because the increased hydrostatic pressure at the bottom of tall tanks protects seahorses against depth-related conditions such as Gas Bubble Syndrome (GBS).
For starters, as a general rule of thumb, seahorses require at least two full body lengths of vertical swimming space or breeding and reproduction may be hindered. For large species such as Hippocampus erectus, which can reach a length of over 8 inches when fully grown, this means that breeding pairs may require a minimum of 16 inches of unencumbered vertical swimming space in order to mate comfortably. That 16 inch figure does not include the depth of the substrate or an air gap at the top of the aquarium, so for all practical purposes, you’ll want an aquarium at least 20 inches tall for your herd of H. erectus.
Seahorses need adequate 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. Typically, the shallower the tank, the more difficult coitus is to achieve and the more likely it becomes that eggs will be spilled during the transfer. Eventually this can reach the point where entire clutches are being lost, which is when most pairs cease trying and no longer attempt to breed (Giwojna, Jan. 1999).
Worse still, shallow tanks increase the danger that an overripe female may become egg bound. In a tank with inadequate water depth, a courting female that has hydrated her clutch may be unable to make the egg transfer, yet she will be reluctant to dump the eggs while a receptive male is still standing by eager to receive them. If she is overly optimistic and retains her clutch too long, hoping to pull off the tricky egg transfer despite the lack of depth, she may become egg bound. Her lower abdomen will become very swollen and prolapsed tissue may protrude through the vent as the pressure builds up. If she is unable to release the eggs at this point and relieve the pressure, death will result. An egg-bound female is thus a very serious complication of depth limitations (Leslie Leddo, pers. com.)
Shallow tanks can be equally hazardous to the health of breeding males. If the tank is too short, male seahorses may not be able to get enough pumping action in as they ascend and descend during courtship displays and mating (i.e., during the copulatory rise) to flush out their pouches and cleanse them properly (Cozzi-Schmarr, 2003). This can contribute to bloated pouch, a type of pouch emphysema.
Worst of all, shallow aquaria leave seahorses prone to various forms of Gas Bubble Syndrome (GBS), which is a fatal condition if untreated. To understand why this is so, we need to consider the relationship between water depth (i.e., hydrostatic pressure) and the solubility of gases. Let me briefly explain:
The deeper you go and the greater the water pressure, the more dissolved gases the water (and one’s blood) can hold in solution. By the same token, the shallower you go and the less water pressure there is, the less dissolved gases the water can hold and the more likely gas is to come out of solution and form gas embolisms in your (and your seahorse’s) blood and tissues.
This means that there is a definite relationship between Gas Bubble Syndrome (GBS) in seahorses and aquarium depth. To put it in a nutshell, the shallower the water depth, the more likely GBS is to occur. There is considerable evidence that tanks 3 feet deep or more provide significant protection against GBS. This is because the gas emboli that cause GBS form more readily at reduced hydrostatic pressure, and will go back into solution again if the hydrostatic pressure is increased sufficiently, and obviously the deeper the aquarium the greater the hydrostatic pressure at the bottom of the tank.
In short, Gas bubble syndrome is a much greater risk to seahorses kept in shallow tanks. For this very reason, large-scale breeders and public aquaria that display seahorses often prefer to keep them in aquaria that are 36 inches tall or even higher. This is impractical for the home hobbyist, but suffice it to say that an aquarium 20-24 inches tall is desirable for your H. erectus for all the reasons mentioned above. When shopping for a seahorse setup, it’s best to opt for the tall or high model of the largest aquarium you can reasonable afford and maintain.
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. For novice aquarists and those new to seahorses, maintaining the best possible margin for error is crucial for long-term success.
Best of luck with your herd of Hippocampus erectus, Don!