Köi-Lö $##$$@$##$$.#$$ wrote in
:

Some people claim toxic gasses accumulate in the gravel, especially if
no live plant roots are running through it. I do have live plants in

The toxic gasses arise from some types of anaerobic bacteria. It is not
especially common, but it is a potential problem if you have a very dirty
or very deep gravel bed. Using gravel heating cables will generate a
natural rising water flow and prevent the development of these gasses.