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Gravel Debate - What Exactly Is the Debate?
FACT: If you took four to six inches of coarse gravel and put it on the bottom of a pond that did not have fresh natural inflow of water, and then fed the fish occupying that system heavily for 1-3 years, enough mulm or wastes would collect IN the gravel to begin to grow hazardous bacteria. These hazardous bacteria, deep in the gravel bed, would eventually become numerous enough to produce measurable quantities of a toxic gas called Hydrogen Sulfide. Article: What you should know about Hydrogen Sulfide. Can Hydrogen Sulfide be produced in one inch of coarse gravel in a Koi pond? Answer: No. There are several reasons, first of which is the almost constant sifting of the gravel by Koi of almost all sizes. Secondly, it's hard to deprive one inch of gravel of oxygen in a pond. Remember, it is only in a total absence of oxygen that hydrogen sulfide producing bacteria can ply their trade. One of the things we're going to do during warm weather in Georgia is test our Aquascape installation for the production of Hydrogen Sulfide gas. Since H2S gas is not rapidly produced, this could be a year or two away from yielding results. Can Hydrogen Sulfide be produced in one inch of sand undisturbed in a fish tank? Answer: Yes. It can, I've seen and measured it. The reason is that tropical fish don't sift their substrate media as much as Koi and Goldfish do. Secondly, sand is a highly compacting media and is prone to favor the growth of Hydrogen sulfide gas producing bacteria. Many hobbyists who like sand in their fish tanks will use a Horseface Loach to keep it turned over. Works excellently. What's the difference? Answer: As you can tell above, different medias can go anaerobic at varying depths and compactness. Sand will compact and proceed towards anaerobic conditions more easily than gravel. More testing is needed to confirm the theory that Aquascape ponds improperly installed with 6-7 inches of gravel are liable to produce Hydrogen Sulfide as so far, the noxious gases concept has been a probable theory, and never proven. We intend to see about that. So how does that apply to my Aquascape pond? Answer: If your pond was installed correctly, it will have less than two inches of coarse gravel on the bottom. If the pond was IMPROPERLY installed, the pond will have more than two inches of gravel in the pond. One of the most common reasons for "excess gravel" in an Aquascape installation is that the installer might bring more gravel than needed. Instead of burying or removing the gravel to another site, they may simply "go generous" and put the gravel in your pond, without considering what could happen if deep gravel is neglected for several years. What if I have too much gravel? Answer: It's as simple as measuring. If there's thick gravel on the bottom of the pond, and it's been more than 24 months, you should consider that Hydrogen Sulfide MAY be trapped in the gravel. Fish of any value should be removed before you trouble shoot this problem. After fish removal, stir the gravel and see if you recover any gas bubbles. Test to see if they contain sulfides with a Hydrogen Sulfide test kit. Remove any excess gravel, perform a water change and neutralize any remaining Hydrogen Sulfide with potassium permanganate. What about the Aquascape "plant pockets"? Answer: I don't have any tangible or reliable information that those should be any problem however in theory, the depth of the gravel and the presence of organic material could produce conditions favorable for the growth of sulfide producing bacteria. We intend to test the theory in the lab. In the meantime, I would personally consider the plant pockets a potential liability, and would recommend division of the roots annually during the Big Cleaning if you want to avoid the development of stagnant pockets.
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