The-tanks-I-do-this-to-are-a-20-and-a-100-gallon-and-I-use-undergravel-filters-on-them......The-gravel-in-the-20-gallon-is-flourite-and-the-100-is-medium-sized-round-pebbles-and-both-are-about-4-inches-deep......I'd-try-the-cleaning-method-I-suggested-and-bleach-the-artificial-plants.....Then-the-Ottos-will-have-a-head-start-and-also-you'll-have-less-nutrients-in-the-water-which-does-lead-to-excess-algae-growth.....I-find-that-even-with-minimal-feeding-I-get-a-lot-of-crud-ending-up-deep-in-the-gravel.......Good-luck.

I-have-another-tank-that's-heavily-planted-with-Eco-Complete-substraight-and-I'm-leaving-that-gravel-alone-hoping-the-plant-roots-take-care-of-whatever-is-in-there.