I am writing this because it is something that comes up so often.
Ich is a protozoal infection that afflicts fish and can rapidly kill them, most often by damaging gill tissue.
The organism goes through a life cycle of;
a small white spot feeding on your fish,
which drops off to the floor of your tank and encysts,
while encysted it divides into up to 2000 new mobile organisms,
the cyst ruptures, releasing the mobile organisms which seek out a host.
Only the mobile stage is vulnerable to treatment by anything that will not also kill your fish.
Here is an old fashioned but very effective method for treatment.
It can be used for most fish but morymids, corys, and some pims are sensitive to salt. Because the organism infest the tank, the whole tank should be treated.
Raise the temp of your tank to at least 85-86 degrees F.
Add aquarium salt (dissolved in water) at a ratio of 2 teaspoons of salt per gallon of water in your tank.
Now wait, while waiting it does not hurt to add a powerhead or airstone to increase the O2 level.
Over the first couple days your fish will look worse but then they will clear up. about the sixth day they will look clear but because some ecystments have not yet hatched keep the treatment up for the full 10 days.
If you are not able to raise the temperature you need to extend the treatment, at 85 the ich's life cycle is quick but at 72 it can take weeks, below 70F treat for 6 weeks.
There are medicines you can use but many fish are sensitive to them, you can also try just heat at 90-91 degrees F but some fish can't take that heat and some strains of ich can survive it.
The salt and heat method is one I have used several times with sucess.
