It’s great that you have a hospital tank/quarantine tank set up and ready in case you need. You bet — I’ll give you a thumbs up on your plans to change half of the water in your hospital tank and then add it to your 49-gallon seahorse setup. Unless your hospital tank is cycled and has a biofilter already established, you will need to make regularly water changes in order to maintain the water chemistry in the quarantine tank while the seahorses are in residence. That would be the only way to prevent almost you spikes from happening in the hospital tank as well.
With all that live rock, I should think that your main tank already has plenty of beneficial nitrifying bacteria, but if you would like to give it an extra boost in light of what has happened, then you might want to consider adding some BioSpira.
Go to a well-stocked fish store as soon as possible and obtain some Bio-Spira and add it to your seahorse tank according to instructions. Bio-Spira is a product offered by Marineland which contains the live bacteria necessary to convert ammonia and nitrite into harmless nitrate. It is available for both freshwater and marine aquariums, so of course be sure to get the Bio-Spira for saltwater. Just use it as explained below and it should prevent the small amount of ammonia that is in your aquarium from harming your Mustangs in any way:
BIO-Spira is a "live" bacteria culture that is sold refrigerated and must be kept refrigerated until used. It can not be overdosed. Repeated dosing of your aquarium with ammonia removing liquids (such as BIO-Safe, Amquel, Ammo-lock and Aqua-Safe) can inhibit the beneficial action of BIO-Spira. Ammonia removing liquids should only be used to initially treat tap water. It is normal to have a small (<2 ppm) amount of ammonia or nitrate during the first few days after set-up. These concentrations are not harmful and will quickly drop to zero with proper use of BIO-Spira.
DIRECTIONS FOR USE:
Shake well before each use. Use 1 ounce (29.6 ml) of BIO-Spira per 30 gallons of water. BIO-Spira cannot be overdosed. Keep refrigerated. Be sure to shut off any UV sterilizers and remove medication by means of a water change or activated carbon.
I would try the BioSpira before anything else, Richard, and if it brings the ammonia levels back down to zero, then you can return the seahorses to the main tank and their metabolic wastes will feed the beneficial bacteria in your biofilter. I believe they would be better off in your main tank with all of the live rock and the lot more water volume than they would be in the 10-gallon hospital tank for an extended period.
If you want to toss a piece of shrimp in the tank to decay a long with the BioSpira in order to make sure that you have an adequate population of nitrifying bacteria, that’s okay, sir — just don’t leave the Mustangs in the hospital tank with no filtration for too long.
Best of luck eliminating the residual ammonia in your main tank, Sean!