That’s an excellent point Kelly makes about the height, and therefore the water depth, in a seahorse tank. Your 20-gallon aquarium is a very nice upgrade for your seahorses since it is twice as spacious and will provide double the water volume for increased stability, and the move should go smoothly without being too stressful for the seahorses at all, but a 20-gallon Long aquarium is lacking in the height that is so desirable for a seahorse setup.
As Kelly pointed out, the height of the tank is important because seahorses need 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 tank that’s only 12 inches tall can be problematic in that regard for large seahorses such as Mustangs and Sunbursts (Hippocampus erectus).
More importantly, a tall aquarium can also help protect the seahorses from depth-related health problems such as bloated pouch and certain forms of Gas Bubble Syndrome (GBS). This condition is caused by the formation of gas emboli within the blood and tissues of the seahorse, and in all its various forms, GBS is the most common health problem seen in seahorses that are kept in small closed-system aquaria. If untreated, the gas bubbles worsen and the condition is fatal.
Tall aquariums are preferable for our amazing aquatic equines because the greater hydrostatic pressure at increased depth is known to protect seahorses against GBS, whereas the reduced hydrostatic pressure in shallow aquaria is known to be conducive to GBS. For this reason, it is always desirable to select an aquarium at least 20 inches tall (the taller the better) for a seahorse setup.
So while your seahorses will certainly fare better in your 20-gallon aquarium, Harry, and I certainly encourage you to make the move and upgrade them into a bigger tank, it would be even better if the new tank was a 20 gallon Extra-High All-Glass Aquarium (20"L x 10“W x 24“H) rather than a 20-gallon Long aquarium.
Best of luck upgrading your seahorse tank and relocating your seahorses, Harry!