I have heard of reverse flow UGF's and I know people that use it. The main idea behind it is the flow of the debris. Usually in a regular UGF the debris gets stuck in the substrate/gravel to which someone would have to use a gravel vacuum to remove the debris. In reverse flow the debris is pushed from the substrate/gravel and into the water column so that their HOB filter or canister filter can catch the debris. Others use it for a reverse flow of water through a well planted aquarium. Thinking it would create more water flow through the roots of live plants. I don't think it's nessasary to use the UGF period but its up to the aquarist.
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