Algae. It grows on rocks that are in the woods due to the dampness the forest, and the New England weather offers it. Scrubbing all visible pieces doesn't mean a tiny unnoticeable portion won't be missed and then reproduce dramatically in the aquarium. That's why I was wondering if boiling would kill all algae.Find on ground, rise off, add to tank. What do you think you're going to introduce from an inert rock?
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when I read this, but it's exactly what I've done in the 15years I've been keeping fish.