How many inches of subtrate in a 20 gallon long tank?

i think the recommended height for ANY tank is 2" - 4" inches (from rexgrigg.com)

to find out how much you need, go here: http://www.plantedtank.net/substratecalculator.html and input your dimensions and such

I usually aim for an average of three inches, but since a 20 long is only 12 inches high, I would recommend 1.5 at the front sloping to 2.5 at the back. Just my two cents.

if your just going to plant some HC/hairgrass or other rosette plants. substrates down to an inch is enough, but if you plan on planting anything else, i'd recommened at least 2 inches.

3-4 inches would depend on the height of your tank too. too thick a substrate will make your tank look a little short and limit the growth of the plants.

Is there a problem with having too much substrate in there? I think I have about 4-5" of soilmaster because my cichlids like to dig and because the substrate is so light.

i don't think there's a problem with that much substrate. just less water for swimming and less height for plants to grow...