In-Pond Gravel Filter - Please Help

I have an in-pond gravel filter that served me well for many years but I decided to clean the bottom from leaves after a storm.

Dropped the level to knee height and raked all the leaves with my fingers and filled it back up again. Time for a good water change anyway. This morning all my fish are dead! A collection I build up over the years. I am devastated to say the least.

I need help to try and understand what could have gone wrong. 


So sorry to hear of your loss. You stumbled onto an important fact a static filter beds. 

Here is an article about hydrogen sulfide poisoning and could explain what happened when you stepped onto you filter bed. The unintended consequence of your cleaning operation can be severe.

Hydrogen sulfide 

Toxic hydrogen sulfide is a silent killer that often goes unidentified as the real cause of fatalities or sick pond fish. Never disturb a biological filter bed without rinsing and flushing the chamber. Beware of walking on an in-pond gravel filter bed!

Sulfur is excreted in the form of sulfide by certain bacteria under anaerobic conditions. Pond bottoms, filter bottoms and other areas where organic wastes settle are particularly prone to sulfide production. These sulfides present themselves in different forms in water. The lower the pH and the lower the temperature, the more hydrogen sulfide will be in solution.

Hydrogen sulfide is extremely toxic for fish particularly when present in the pH range 5,0 to 8,0 where it will interfere with respiration at cellular level to cause symptoms of hypoxia (oxygen shortage). The fish will increase their ventilation rate and become more active in an effort to find an escape route from the toxic environment. The toxicity increases with decreasing DO concentrations. It is lethal to adult fish exposed to 0,04 mg/l. Consider any detectable concentration of hydrogen sulfide to be detrimental to fish health in the long term.

Good pond hygiene and regular maintenance will eliminate the chance of hydrogen sulfide accumulation. Any disturbance of the biological filter bed should be flushed away immediately. Sand filters that are backwashed, should be rinsed properly. Flush the bottom drains regularly and remove uneaten food, excrement and dead organic material. Clean the filters at least twice a year. Regularly remove all sludge buildup.32, 33, 34, 35

Other than a major water change and vigorous aeration of the pond is about all you can do to solve the problem of hydrogen sulphide poisoning. The oxygen in the air oxidizes the sulfide into harmless sulfate.

The formation of hydrogen sulfide will be limited or even prevented by running the pond at a higher pH (7.5-8.3) and with sufficient calcium ions in solution. This can easily be achieved by operating the pond water system with sufficient of a product like StabHi pH Pebbles in the water system.


(Extracted from Living Jewels (1996) by Ronnie Watt and Servaas de Kock, updated 2004, 2018.)

Reference material sited:
32. Boyd, C.E., 1990
Water Quality in Ponds for Aquaculture
Birmingham Publishing Co.

33. Boyd, C.E., 1982
Water Quality Management for Pond Fish Culture
Elsevier Scientific Publishing Company.

34. De Kock, S., 1994, 2017
Personal observation.

35. Yamada, R., in Lannan, J.E.; Smitherman, R.O. and Tchobanoglous, G. (Editors), 1986
Principles and Practices of Pond Aquaculture
Oregon State University Press