When I still had my ‘’top-in-top-out’’ filter system and used shading net as filter media, I had the same scenario. Many mosquitoes, some blood worms, flees, water flees between small stones in waterfall and some funny creatures that looked like crickets without legs and a ‘ flat- scorpion- look- alike ‘ tail. (Still don’t know what this was) I can not blame the shading cloth but rather the stagnant water that I created with my brilliant ‘to-be found- at-my-home-only’ filter system.