I can tell you first hand when I had a parasite break out & in a general method of prevention, you should lower the salinity below .021. I don't know if you have inverts (crabs, snail, etc), because they could be effected by the salinity drop. I would suggest you to do this to remove any possible disease or parasite. This should be done over a period of 5-6 days, dropping the salinity by 2 or 3 points per day. Say your salinity is at .021, then the first day drop it safely to .018, the second day down to .015, third .013, fourth .011, & the fifth lastly to .009 (your goal). I know this sounds outrageously low & I thought so too, but ich (if that's what you have), most diseases, & parasites can't live at a salinity level this low.
Now if you have another tank & I suggest you get a 10 gal if you don't for new arrival isolation, put the inverts in there while you do the salinity drop. If you don't have another running aquarium (heat, pump, lighting) & all you have are snails & hermit type crabs (blue, red, etc) than a container like a bucket with some sand &/or a piece or two of live rock should work. If you have an extra heater & you should, use it to regulate the heat, but I have had inverts [snails & crabs] live in buckets while I was moving for weeks (shhh). Leave the container in a close-to-the-tank temperature room or part of a room away from windows & vents, if you don't have a heater. My fish started to reacts immediately towards this action...in a good way. This procedure should be done for 30 days (I went 45 to be safe). I would say if you don't have coral & inverts & don't plan on purchasing any in the future, keep the salinity below .021, but not as low as .009. I keep mine safely at .019 & I have red & blue hermit crabs. I note that you have mushrooms & I also am about to receive a shipment from Saltwaterfish.com with with a mushroom & Anemone in it & will see how they respond to a salinity reading of .019.
Also, post in the Disease & Treatment board, you should get some good advice & good luck.
