How deep is your gravel bed.

1 inch in the front to 2-3" in the back. it's best to strike a balance, i think, between a bed that is deep enough to be beneficial to plants and yet not so deep as to encourage anaerobic bacteria. it also depends on the size of the gravel. some of my tanks are also sand-based, so i keep those shallower than bigger gravels.

A very general rule of thumb is 1lb gravel per gallon.

im are about 1" when put in but that dont last long when you have a americans lol some place are around 3" some their is none but i flatten it out every week during the water change

The deep beds of SW tanks are effective because they are made of sand. With the small particle size, the anaerobic areas develop easily and are stable. With larger size substrate--including crushed coral--you don't really get the conditions needed for the nitrate consuming bacteria until you are much deeper than is feasible (think 18+ inches was found to be effective). The gravels used in most FW tanks won't support anaerobic bacteria unless they are covered--ie, under large sections of wood or other decorations.

I have tanks that vary from less than one ich up to about 4 inches, in my planted tanks.

For FO tanks I use 1/2-1" of substrate.

For planted tanks I use 3-6" of substrate (dependent on tank height)

I do not slope the substrate unless I build terraces. For me it just does not hold long term.

OG: a number of FW folk use sand (not me), so anoxic areas are quite possible to likely depending on handling and depth, and more than any other single factor - the organic load in the substrate.

2'' of sand in front, ~3.5'' in back, give or take on both.