Yes, tap water as top off is going to raise your nitrates and dissolved solids. Information like that should be included in your first post cause that is probably the cause of your troubles.

As for the filter floss, any kind that you use should be cleaned at least weekly to prevent any build-up of detritus.

Any time you have a problem that pops up, think back to anything you have done differntly in the past few months and that is a good place to start to find the cause.