Have you ever been cleaning your pond, maybe pulling out dead plants, and smelled rotting eggs? This unfortunate smell is hydrogen sulfide, the product of anaerobic bacteria buried deep in the mud. Hydrogen sulfide is highly toxic to fish and can can death very quickly. Fish who are exposed suffer an increase in secondary infections from bacteria, parasites and fungi and possible sudden death.
It is essential to remove your fish from the pond before undertaking any serious deep cleaning, especially into boggy areas with overgrown plants. Set aside a whole day to tackle your cleaning project and put your fish in a temporary tub with an airstone for the day. They will be fine for a few hours without filtration, but no more. Bring in extra help to make sure the project can be completed quickly. If you use a professional company to perform your cleanings, make sure they are aware of hydrogen sulfide and can provide the proper accommodations for your fish. Once all the plant material is removed, the pond must be drained to remove all hydrogen sulfide.