Really need help! Weird black film
So for months I've had this hard black film on the caves and some of the plant leaves in my fish tank. Nothing I do gets rid of it! Chemi-pure, water changes, algae eaters (otos and bnp) Feeding less, snails, scrubbing, another filter additive I've forgotten the name of now.
I've been adding fertiliser by the EI method recently. Hopefully getting the plants healthier would help. They were a little yellow.
I'm not sure what else to do. I do weekly 50% water changes and use prime. The substrate is black moon sand. It's been set up for almost a year now and was doing so well.
The filters are an aquaclear 70 and a fluval 305. It also has a maxijet pro in the utility pump mode for more flow.
Houses to apistos, 2 flag cichlids, 2 phantom tetras (lost 2 when I accidently dosed them with chlorine, and 1 when I was gone as the pet sitter didn't bother to top off the water and it got below the heater.) 6 albino cories, a bushy nosed pleco,5 kuhlie loaches, 2 nerite snails, 12 neon tetras, and 2 bolivian tetras.
the tank is 80 degrees.
Any advice is appreciated.