Estefano is right on the button with regards to quarantining marine fish and invertebrates. For marine fish, it’s customary to maintain hyposalinity in the quarantine tank, which will eradicate most protozoan parasites and ectoparasites, and then simply maintain the fish for several weeks to see if any health problems manifest themselves in the meantime. The hyposalinity helps reduce stress by making it easier for the new arrivals to osmoregulate, and since most bacterial and fungal problems are secondary infections, controlling ectoparasites that attack the skin and gills of the fish by using the hyposalinity can also help to minimize problems with bacteria and fungus.
For marine fish other than seahorses, reducing the specific gravity to 1.018-1.010 is customary, whereas for seahorses, a specific gravity of 1.011-1.012 is appropriate in the hospital tank if you have an accurate means for measuring SG such as a refractometer. If you will be relying on a pet shop hydrometer to measure the specific gravity, then it safer to aim for 1.015 for seahorses in a hospital tank in order to provide a suitable margin for error. If you do a search on this forum for "hyposalinity" or "osmotic shock therapy," you’ll find lots of more information explaining exactly how to administer hyposalinity safely.
Other than adjusting the specific gravity in the hospital tank to a level that’s appropriate for hyposalinity, you are primarily watching the marine fish to see if any problems develop. A quarantine period of 4-6 weeks is usually recommended, which is sufficient time for the incubation period of most diseases to manifest themselves. Should health problems crop up in the hospital tank, they can then be treated safely with the appropriate medications without any risk of introducing any sort of pathogens or parasites to your main tank.
While they are in quarantine, it’s appropriate to deworm wild seahorses with Panacur and to treat them prophylactically for internal parasites using praziquantel or metronidazole, and for any external parasites they may possibly be carrying using formalin bath(s) and/or freshwater dips. See my earlier post to Estefano for the recommended quarantining protocol for wild seahorses. It’s available at the following URL at the end of the discussion thread:
Of course, if the new arrivals are captive-bred-and-raised specimens from a High-Health aquaculture facility such as Ocean Rider, then the quarantine protocols required for wild seahorses are unnecessary. Your Mustangs and Sunbursts will be free of pathogens and parasites when they arrive and will reach you well-fed and in top condition, so they can be introduced directly into the main tank without any need for quarantining.
As Estefano pointed out, it usually isn’t necessary to enforce a strict quarantine on snails and most other invertebrates that are going into a seahorse tank. For one thing, invertebrates in general are not susceptible to the same pathogens and parasites that plague seahorses and other marine fishes. If they were carrying any of the parasites that could bother seahorses, it would be as hitchhikers, and that’s unlikely because those same parasites normally cannot survive long without a suitable fish host. So there is relatively little danger of introducing seahorse parasites via the snails we use as sanitation engineers.
Secondly, snails and invertebrates in general cannot tolerate the usual prophylactic measures we apply to marine fishes when we quarantine them. For example, many types of snails cannot withstand hyposalinity let alone a freshwater dip. Nor do they tolerate the usual chemi-therapeutic agents we normally use to cleanse quarantined fish of parasites when they crop up, such as formalin, malachite green, copper sulfate, dylox, Panacur, etc.. So there would be very little we could do to treat snails or other invertebrates prophylactically or preventively even if we quarantined them indefinitely.
As for myself, I normally don’t quarantine the snails and hermit crabs for my cleanup crew. Rather, I screen them visually to avoid snails (or micro-hermit crabs) with telltale fuzzy pink patches on their shells indicating hydroids or that may be carrying nuisance algae. If in doubt, I’ll use a clean toothbrush to scrub their shells clean and then carefully acclimate them to the aquarium. And, of course, I try to avoid getting any of the water from their shipping bags into the main tank.
In principle, waiting several weeks after you add the cleanup crew before you introduce any fish to the aquarium is a sound safety measure. The idea is that if the sanitation engineers happened to be carrying any sort of parasites when they were added to the tank, those parasites will perish during the waiting period without a vertebrate host to sustain them. Waiting for six weeks will allow sufficient time for the complex life cycle of parasites such as Cryptocaryon irritans or Amyloodinium to run its course, and it is a wise precaution to help assure that even these stubborn parasites don’t get a toehold in your seahorse tank. It’s always better to be safe than sorry, and the invertebrates could be disease vectors if they were maintained in holding tanks that share a common filtration system with wild caught marine fish, as is a common practice at many wholesalers and retailers.
In my opinion, the risk of introducing parasites or pests other than hydroids, which are not a problem for large seahorses, along with your cleanup crew is relatively slight — similar to the risk you take when you introduce macroalgae to your tank or when you provide your seahorses with live foods such as ghost shrimp, Gammarus amphipods, copepods, post larval shrimp, Artemia, live Mysis, and so on. Ordinarily, most hobbyists don’t quarantine live plants or live foods before we add them to our aquariums, but if you want to be extra safe, waiting six weeks after you add your cleanup crew isn’t a bad notion at all.
If you do introduce the cleanup crew and macroalgae to your aquarium and then wait up to six weeks before introducing seahorses or any other fish, all you need to do is wait and count the days. And you’ll need to feed your sanitation engineers every two or three days, since there will be little for them to scavenge in a newly established aquarium.
Best of luck with your new seahorse setup!