Ok I hope not to stir things up a bit but I don't think this is a parasite problem. Fish are like humans in a similar way. Think of the last time, you switched on your room lights (or had someone else do it on you) and did not squint your eyelids trying to adjust to the brighter lights. Fish are similar except they don't have eyelids so they try and make themselves more comfortable by darting to a less bright place. Taking the lid off causes things to get dark again similar to a large predator's shadow overhead and I am not surprised the fish are getting scared. If these were my fish, I'd stop with the medication crap and just do my daily feeding routine and weekly waterchange routine. Let them get to know me and feel comforatable with me. Give them some plant cover (maybe an amazon sword) to protect them from the bright lights and over time they will start to become more social with you.