Do you have any decor that could be leaching out into the tank? You said that you don't have driftwood, but do you have anything else in there that could be discharging tannins or the like?
If not, it's probably algae. That isn't unusual in a tank at 6 weeks. Scrape it off any surfaces it bonds to for now. Don't start dumping in algaecides just yet.
|