H2S

[Jimmy74]

How many inches of sand would be too much and put out levels of hydrogen sulfide that would be dangerous for fish?

 

[AbbeysDad]

The beauty of sand is that organic matter does not get down under to decompose anaerobically and create noxious gases... It is important NOT to stir the sand so organics get buried down under!
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I have about 3" of sand in several tanks that I don't touch (although I might rarely siphon the surface if need be).
If you will have rooted plants, don't use a really fine sand that can pack and inhibit root growth. I like pool filter sand, but like my friend Byron, I have a couple of tanks with regular play sand from the big box store.
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Note: Charles Clapsaddle of Goliad Farms is a professional breeder in South Texas. His breeder vats have as much as three (3) inches of mulm that he's convinced is beneficial.