Getting a good liquid test kit is essential. We need to know what your parameters are to give better advice. If you can, take a sample to your LFS
and make them use a liquid test kit to test it. They should be more than willing to help you.
If the fish still have Ich I would slowly raise the temperature of the tank to 86*F and keep it there for two weeks after the last white spot has disappeared. Then slowly lower the temp back to normal.