It sounds like your Mustangs are suffering lingering effects from shipping stress due to the combined effects from depressed pH and ammonia build up in the shipping water. This can happen during shipping or during the acclimation process, and is fairly common following long-distance shipping, particularly when overnight delivery is not possible, as in your case.
From the symptoms you describe, it appears your pregnant Mustang is still recovering from the toxic effects of ammonia exposure. The most obvious symptoms of ammonia poisoning are a loss of equilibrium, hyperexcitability, increased respiration and oxygen uptake, and increased heart rate. At extreme ammonia levels, fish may experience convulsions, coma, and death. Seahorses exposed to less extreme ammonia levels will struggle to breathe. They will be lethargic and exhibit rapid respiration. They may appear weak and disoriented, periodically detaching from their hitching posts only to sink to the bottom.
That’s what I think has happened to your Mustangs, Carrie — moderate ammonia exposure at a level less than constitutes ammonia poisoning. You mentioned that the pregnant male was sitting on the bottom, breathing heavily. This sounds like the sort of labored breathing, disorientation and loss of equilibrium that results from exposure to high levels of ammonia. The female Mustang also seems to have been affected, albeit to a lesser degree. The male was probably a little more susceptible due to the demands of his pregnancy.
This type of shipping stress and ammonia exposure is completely reversible providing the seahorses weren’t exposed to toxic levels for too long, and the best first aid you can provide for ammonia poisoning is to immediately transfer the seahorses into clean, well-aerated saltwater with zero ammonia and zero nitrite. It sounds like you already accomplished that when you introduced them to their new aquarium, Carrie. All they need now is a quiet, stress-free environment and time to recuperate.
Here’s what I’d like you to do today, Carrie: increase the aeration in your seahorse tank (add and extra airstone if necessary), turn off the aquarium reflector and continue to leave the tank darkened, and give the seahorses as much peace and quiet as possible while your Mustangs recovers. Just provide them with a stress-free environment and leave them alone to recover at their own pace. Add some "feed-and-forget" live food that will survive indefinitely in a marine aquarium until it’s eaten, such as the red feeder shrimp from Hawaii (Halocaridina rubra, a.ka. Volcano shrimp) or live adult brine shrimp, but otherwise don’t pester them.
Then report back to me and update me on their condition tomorrow morning. If provided with good water quality and stress-free conditions, your Mustangs may well show dramatic improvement and perk up considerably overnight. Provided you acclimated them properly and your aquarium has zero ammonia and zero nitrite, they should recovery fully.
If you’re pregnant male is not much improved and is still breathing rapidly (compare his respiration’s to the Sunbursts that seems fine) when you report back to me, we can consider treating him with methylene blue in a hospital tank. Commonly known as "meth blue" or simply "blue," this is a wonderful medication for reversing the toxic effects of ammonia and nitrite poisoning. Methylene blue transports oxygen and aids breathing. It facilitates oxygen transport, helping fish breathe more easily by converting methemoglobin to hemoglobin — the normal oxygen carrying component of fish blood, thus allowing more oxygen to be carried through the bloodstream. This makes it very useful for treating gill infections, low oxygen levels, or anytime your seahorses are breathing rapidly and experiencing respiratory distress. It is the drug of choice for treating hypoxic emergencies of any kind with your fish. However, methylene blue will destroy nitrifying bacteria so it should only be used in a hospital tank (if used in an established aquarium, it will impair the biological filtration and the tank may need to be cycled all over again).
I’m confident none of that will be necessary, however. I’m betting your Mustangs will be good as new in the morning; under these circumstances, they almost always are. So just relax, try not to worry, and keep an eye on them from afar.
Your red feeder shrimp should be fine in the clean 5 gallon plastic bucket, Carrie. A heater is not necessary, but you want to provide them with some aeration. Just set them up as follows:
These fabulous little feeder shrimp can be kept indefinitely in a spare 2-10 gallon tank, or even a clean, plastic bucket, that has been filled with clean saltwater and equipped with an airstone for aeration. Neither a heater nor a fancy filtration system is required. They thrive at room temp and reduced salinity (1.015-1.016), and all they require is an airstone (or a simple air-operated foam filter at most) to keep the water oxygenated, with perhaps a little coral rubble as substrate and a clump or two of macroalgae (sea lettuce, Ogo, Gracilaria) to shelter in. They’re easy to feed — they feed primarily on algal mats and bacteria — but they will accept vegetable-based flake foods and pellets such as various Spirulina products. They are filter feeders and can also be fed with yeast or commercially prepared foods for filter-feeding invertebrates. Many people find an easy way to feed them is to place a small piece of algae-encrusted live rock in their holding tank; once they clean it off, simply replace it with a new piece of algae rock.
Best of luck with your new seahorses, Carrie! Here’s hoping they’re all good as new by next morning.