You're not the only one ..... I killed 45 goldfish when I first started ...... hopefully, someone will come on that has a good idea of what the problem might be ...... I have not had any problems at all with my Tilapia.

Here are some links that JCO has provided;
Clik Tilapia Diseases & Cures

Looks like badflash and I were writing at the same time He beat me to it