Tilapia Diseases 101

Tilapia tolerate adverse water quality and other stressors better than most other commercial aquaculture species. Because stress and environmental quality play such important roles in the disease process, tilapia are labeled as being very "disease-resistant." This basically means that in the presence of pathogens, tilapia are the last to break with disease.
As a result, tilapia growers worldwide did not historically practice clean culture methods. Moreover, they did not generally implement the biosecurity measures that had become standard in industries that grew less disease-resistant fish such as trout and salmon. In other words, there was no apparent penalty for being careless - or so it seemed.


Read more:  http://www.americulture.com/Disease.htm

When fish are sick I generally add enough salt to bring the concentration up to 3.0 ppt.
While the fish stores may try to sell you various cures, my experience is if it takes anything more than salt your fish were probably doomed to die anyway. 

Some people go higher then 3.0 ppt, but that's what I use.  I have a meter so it's pretty easy to be accurate but you can go by weight.  For example if you have a 100 gallon system you will add about 2.5 lb of salt.  I did a little math.  5.5 cups = 2.5 lb.

So get some plain Solar water softener salt and dump in 5.5 cups for each 100 gallons of water in your system.  Toss it in the sump tank so as not to hurt the fish.  You can change out some water to bring the level down to about 1ppt after the fish stop dieing.  You should not leave the salt that high forever because it's not good for some plants and the pathogens you wish to get rid of can adapt to the high salinity, but a little is good.  I like to use Sea-90 to keep trace minerals in the system.. You could use Sea-90 instead of Solar water softener salt, but it's more expensive.