Look up cyano it sounds like you have the same thing going on in your tank as I am with mine. Mine is just turning a year and I am not the only one fighting it in my area. Do pwc and try to siphon off as much as possible. Doing a couple of smaller water changes seems to work best for my tank instead of one large one.
Thanks for all the advice guys. I've done one pwc and plan on another one next week. Could this stuff be the cause of my mexican turbo snails dieing off? I lost four now and one I've had in the tank for months the other three about four weeks.