Posted 04-07-2004 at 05:50:09
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We run a bio filter with our pond, and have with our various ponds over about a dozen years or so. Your idea will work, but 15 gallons won't make that much of a difference. Our current bio filter is about 35 gallons, and I think that's about the smallest that'll be effective.
Ours has a standing pipe through the middle, water is sprayed into the top of the pipe, out through holes in the bottom of the pipe, then percolates up through a layer of coarse gravel, then pea gravel, then some fine commercial filter media. Finally, we float water hyacinths on the surface, just prior to the water running downhill back into the pond. We've filtered up to 1,000 gallons with several koi and many goldfish with this setup. It takes about two weeks for the bacteria to establish in the filter at the start of every season, but after that it keeps things crystal clear and aerates and cools the water pretty well during the hottest summer months.
I drain a few gallons from a drain in the bottom weekly to get the muck out, and use that as fertilizer in the garden. I pre-filter the pump through a coarse strainer just to keep out the stuff that'll stop the impeller.