Fish will be fine. One thing you can do is while you are doing your maintenance, is lift the diffuser out of the water, drain off any water on top of the disk and then pour on some H2O2 and let it sit while you complete your maintenance. When you are done, just dip the diffuser a few times in clean water and put back in the tank. This should keep it nice and clean (one thing I haven't been doing but will start with the bottle I just put on yesterday).