"all skimmers perform best in 8" to 10" of water" ???

Ya, you have a 1" drain and a 3/4" return bulkhead fitting. See how the return line comes up and goes through the "knockout" on the overflow wall. If you do not have a siphon break you will back flow to as deep as the return goes in your tank. Most folks drill a hole at the 90 that your line-loc is screwed into. Make sure that stays clear and unclogged and is big enough to allow a siphon break.

You will back flow the portion of your tank that is above the bottom of the overflow grate holes. Probably like a half inch, and anything your returns siphon. It is a manageable amount for a 20g, you just need to make sure to get it figured out and that you allow enough room for all that.