I have pool filter sand and big plecos with an fx 5 and it is about 3 to 4 inches from the bottom and have never had a problem
What is the stocking? This is important too!
The most contaminated water in aquariums is at the surface. Sure there are feces and debris on the bottom, but the air water interface is where most dissolved and semi dissolved organics accumulate. This is why I prefer overflow/surface skimming, and simply vacuum the other stuff. If your filter picks up the debris from the bottom, (unless you clean the filter daily) it isn't gone, it's still in the water column as the water in the filter passes over it, just a bit out of sight.
The impurities at the air water interface are the things picked up and removed with fractionation.
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