i do regular water and filter changes. its a 55g. .
Regular filter changes? Are you changing the whole filter?
Doing this will cause your tank to recycle just about everytime you change it. I only change filter if i absolutely have to. I rinse off the gook and stuff in old tank water during water change day. Most of your bacteria is in there unless you have the bio-wheel...
Ick is very very contagious. No sense in even QT'ing them now you should defineatley treat the whole tank. Get a Ick Cure or some kind of medication that is for ick that has malacyite green in it. Try not to use any MacrynI or II (as it destroys bacterial colinies in the whole tank).
Treat ick for the entire time according to directions on bottle. Even if fish are OK, still treat, otherwise the parasite will become imune to the treatment. Make sure to take out your Carbon of the filter. And turn off lights in aquarium....it usually helps. For now you can add 1 tblspoon of salt per 10gallons. and raise temp up a few degrees. You actually want to reach a goal temp of 82. But not all at once. Salt and raising temps usually helps untill meds are given. Dont use both methods it will be VERY stressful for fish, but will help untill meds can be provided.
It is very important as a hobbist to be able to test waters of our fish. Getting ammonia, nitrItes, NitAtes, and PH test kits are very important so we know whats going on in tank. Amm, Nitrite and Nitrate most important. PH is only important for some species because they thrive in it. Most fish will adapt to our PH in tanks. Please go get a test kit when you can. It really is helpfull. When ammonia is over .50ppm and nitrites reading over .5 water changes need to be made. Right now you should be tsting nitrates because your tank is cycled. However if you are replacing filter every couple weeks with no back up bio media (like the Penguinn Bio-Wheel) you are putting you fish through that cycle over and over and over again.
I hope this helps a little.
(**If you have any kind of loach in your tank Do not add salt. They dont like it.)
I hope everything works out.
Did the fish that did have any salt sprinkles on them?
You may have more than one disease in your tank. Ick is usually the most common. Sometimes we cant control it. However it can be preventable most of the time by frequent water canges, good diet for fish, and good maintence of the tank they live in. Even after all that they still do get sick.