Not sure but the same thing happened to me. I think it is calcium participation. I also used tap water to cycle my tank. Tap water is okay but only to cycle after the cycle be ready for a top off tank and keeping it filled with ro di
only or you will surely have problems. You should be able to buy a algae scraper at any lfs
it has a metal end and if your tank is glass should work good for scraping that white stuff off. Or a razor blade but only if it's a glass tank. That how I got it off my tank glass.
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