Welcome to TRT
Well.......... IMHO, you already have way more rock than you will ever need, and so much rock that it doesn't leave much room for critters. Personally, I'd probably remove about half that rock.
In the sump
, you can use a filter sock. Remember, the whole reason for running this type of filtration is to REMOVE solid particulate matter BEFORE it decomposes and pollutes your water. So, what ever material you choose to use, it needs to be changed out often. In rapid moving, warm water, a tiny piece of fish poo can break down in hours, so media like this needs to be changed out every day or two.
For my system, I have multiple 100 micron socks that I try to change out every day. I have a container in my stand where I place the dirty socks. When I start running low on clean socks, I just dump the container of dirty socks in the washing machine with a little bleach, and I'm good to go until next time.