Look for a spot of dead, not moving water in your tank. Especially if old food has collected there. Another good spot to check is inside hollow decorations, and under decorations where old food might have collected. Any place where it might be possible for a chunk of food to sit for a week or three without being disturbed.
In my case, it was a spot under a piece of driftwood, where a few freeze dried shrimp had drifted in and gotten pinned. I had an actual patch of fungus spreading from the decaying shrimp into my substrate, forming a semi solid mass. After removing the mess and that patch of substrate, my ongoing fungus problems went away with the next waterchange.
I had one previous problem with a fake piece of decor. It was hollow, water went in, then did'nt circulate. Stagnant water eventually becomes sick water, and every time a fish poked his head into the decor, he got sick.
Or it might not be anything you can find. Don't drive yourself nuts looking for something that is'nt there. A little fungus and bacteria live in your water as free floating debris as well. That's what effects most weak or stressed fish.
