Its an urban myth that people have spread around. It happens and it can kill fish but at the end of the day in a planted tank the plants will remove most of it before it gets to the bottom.
Even then it shouldn't be a problem because if it was then there wouldn't be any organic substrates to put under the gravel/sand! They would all have to be thehard grit like substrates like TetraComplete!!.
The back of my tank is 8 inches in some places. If we were to limit the height of our substrates to 4 inches then we would not be able to scape anymore because some of our scapes require these 'mounds'
I suggest you do some research on this subject. The problem has never been suggested to be the substrate.
If using sand then make sure to use 2 inches or less due to compaction. Under the sand then there can be 20 inches of other material. It is due to the sand being such a fine partical size that this anaerobic 'problem' occurs.
This is still normally negated with theuse of MTS and plants anyway.
If you don't believe me then read this (on our very own forum) written by the week respected and incredibly knowledgable nmonks.
You might want to just clarify the "silver sand" bit. There are two grades sold (at least here in the UK), known as 'sharp' and 'smooth'. Sharp sand is unsuitable. What you want is smooth silver sand, and this stuff is usually sold in garden centres for use with various types of plant. It's inexpensive, and usually costs less than aquarium gravel.
I _always_ use silver sand in my tanks. With fish like spiny eels, burrowing gobies, and freshwater flatfish, sand is essential. Puffers and cichlids also seem to like foraging in it.
My plants have always done really well in it. There's a misconception that anerobic conditions in the substrate are bad. They're bad only if the gases build up and come out into the tank. In a well maintained tank this won't happen.
The plants carry oxygen down their roots through special air spaces inside them (that's why bits of aquatic plant root float). This helps keep the substrate less anaerobic that it might be. But the low oxygen concentration also creates "reducing" conditions that shift mineral ions to the forms that plants can absorb. This is the opposite to what an undergravel filter does, which "oxidises" the minerals making them unavailable to the plants, and why plants usually grow badly in tanks with undergravel filters.
Burrowing fish and Malayan snails will remove any food long before the gases build up, and the Malayan snails also help aerate the substrate as they burrow through it.
In ten years of using sand in tanks, often to depths of 7-8 cm, I have never, ever seen or smelled any dangerous gases. Incidentally, I place sand on top of a layer of pond compost to maximise plant growth... this works really well.
So, my thing is this: if the fish and plants like sand so much, why do so many people still use gravel?
Hope this clears this little problem up
Andy
/www.fishforums.net/content/New-to-t...ubstrate-Depth/
