Good news! Just keep treating for a little while longer. Ich doesn't just live on fish it lives in the substrate as eggs and a free swimming larvae that you can't see, then the cyst, then free swimming adult which you can't see which lays eggs in the gravel. It takes up to two weeks to kill them all.
In the diagram below the tomite is the eggs in the gravel, the theront is the first free swimming stage, the trophonts are the cysts and the tomont is the second free swimming stage. The second diagram shows how long each stage lasts at that temperature.
(I really need to make one of these that is all in one)