Cleaning an "all in one" tank

[mrsjoannh13]

I have a UNS 60A which is a 20-ish gallon "all in one" tank. It has a compartment at the back of the tank which houses the filter media and a power head to circulate water over the filter media and back through the tank. Today I was cleaning out the back compartment and took out all of the filter media rinsing out in a bucket of tank water while I siphoned out the back compartment. Previously I had never shone a bright light on the area while I worked. Today I did just to make sure I was getting all the gunk out. Well, I didn't realize there Is a channel (for lack of a better word) that runs under the filter media that I can only get to by sticking a siphon hose down the right side where the power head is. It's kind of hard to explain but the far right side is shaped like a backwards "L" with the bottom part of the L running under the middle and left compartments where the filter media goes. It's basically impossible to get a siphon hose all the way under the bottom channel to suck all the gunk out.

Does anyone else have an all in one tank like this and have any tips on how to get that area really clean? Is this pretty much the best I'm going to be able to clean it given the design? Thanks!