If all you are doing is drainage then you really don't need a charcoal layer--we haven't done that for any others that don't have a water feature. We use the hydroton, mesh screen, and then cocoa fiber mixed with moss, etc. You can also add another layer of fine gravel on top of the screen, but that will add to the weight on the viv. In regards to the CUC, I get very nervous leaving critters in the vivs because a lot of the reptiles I am keeping can be sensitive and some of the feeder insects may bit them (i.e. crickets). I also get worried about bad mites, etc. so I would rather not see anything in there but the feeders we have put in and the reptiles themselves. The feeders only have so long to be eaten and then they are gone. Spot cleaning their waste, removing/replacing dead plants, and wiping down the leaves and glass should be enought to keep things tidy.