Use them as directed, add a bubbler or powerhead for extra O2, most fish fish that are sensitive to them are scaleless, small scaled, or hyper-active. Tinfoills are fairly hardy and even tolerate copper-based meds. I am not sure which flukje med you are using but most are based on either copper or formalin. tinfoils tolerate both at levels that kill most cats and loaches,m once more, follow the package directions, ODs will kill even the most tolerant of fish.