Nitrate reduction in freshwater sand beds

BJP · 12-24-2006, 10:18 AM

We've had some discussion and disagreement in another thread so I went digging this morning for some more information.

http://www.oscarfish.com/-vp581786.html

Quote:

I continue to research this, but from reading the following article I have drawn some conclusions, thus far::
http://www.skepticalaquarist.com/docs/nutrient/sulfint.shtml

It is much more difficult to poison a tank with Hydrogen Sulphide than I originally believed and would require a mistake be made by the aquarist for it to occur. This is based upon the following two excerpts from the article:
Quote:But, pretty rarely in most aquaria, pockets of hydrogen sulfide can also form in deep substrate layers that are never touched by oxygen. In entirely oxygen-free zones of the substrate, de-nitrating bacteria can thrive, stripping the oxygen from nitrate and nitrite. Their activities produce a nitrate gradient. In sufficiently deep substrates, nitrates may become entirely used up. Below the de-nitrating zone, where there is neither nitrate to work on, nor oxygen to interfere, sulfate (SO4) can become the next-best electron receptor for obligate anaerobes, those bacteria who can't handle oxygen at all.

Quote:So, if H2S has formed in a deeper anoxic layer in substrate or a thick biofilm, various aerobic bacteria are waiting to scavenge any available hydrogen sulfide and oxidize it to harmless sulfate. In an undisturbed substrate, bacteria like these would tend to congregate in a thin layer at the limits of diffused oxygen, subsisting on any H2S that might diffuse up from a deeper anoxic layer.

These statements lead me to believe that a sufficiently deep, undisturbed substrate will assist with nitrate reduction via denitrification. In even deeper substrates, hydrogen sulphide may form (only in the absence of nitrate) but will be consumed by aerobic bacteria as it rises from the oxygen free layers of the substrate into layers in which oxygen is available. In order for hydrogen sulphide to escape into the water column the substrate must be sufficiently disturbed allowing H2S to bypass the aerobic bacteria that would otherwise consume it. This could occur by aggressively vacuuming gravel, or perhaps by a digging fish, but it would have to be an area that has not been disturbed for a lengthy period of time.

My conclusions thus far....

1. A sand or very fine gravel bed of 4" would be safe and beneficial to most tanks if left undisturbed. This would probably rule out the use of this technique on many cichlid tanks because of their digging.

2. I don't think hydrogen sulphide will be an issue in substrates 4" or less.

3. With a gravel bed of 4 inches or more only the surface should be vacuumed.

I am seriously rethinking my aggressive gravel vacuuming habits. In my Oscar tank I have the large gravel, which is not conducive to this process, so it will either have to be removed or the aggressive vacuuming continued. In my two 55s I am considering increasing the depth of the substrate to 4" or 5" and only vacuuming the surface of the gravel. In addition, I may add 4" of fine sand to the sump that is connected to the 100 gallon.

BJP · 12-24-2006, 10:22 AM

I'm still wading my way through reading the entire thread but if you have any interest in the topic I think you'd enjoy it also.