I have that in my snailarium (snail growing tank) It is unusual to get it in a tank with fish!
Its a combination of food debris, snail poo & snail trail, low water movement, & stagnant water (although the latter in your case may not be true) It can be accompanied by white stringy worm-like thigs & little triangular cell mites (same size as daphnia but white....look a bit like a mans meat & 2 veg...sorry but thats the best discription.)
You are doing the best thing with the water changes for the stringy stuff but it clings to rocks & the bottom of the tank & only flies freely when a current occurs, (mostly water added or stirred.) It can also be attributed to too much light. Fish only need 8hrs, but i am going by coldwater set up. Hope that helped.