The one thing made clear to me by Dr. Tanner when I began the process of converting to HMFs and Poret cube filters was his "dislike" of filter floss. Here is what he wrote back when I said I was converting Eheim canisters to use Poret and wondered about the final floss pad that would be the last thing through which the filtered water would pass:
To your questions:
I never recommend to put floss in canisters (or any other filter), the stuff just collapses and then clogs the filter much sooner than the rest of media. I would run the canisters solely on 10/20 ppi foams. Start with 10 and then the rest all 20. The goal is to run them as long as possible.
What he explained to me was that in a fully established HMF what happens to the organic particles was that, as they passed through the foam, they are biologically broken down. largely consumed and become ever smaller "pieces" until finally all that is left is a minimal amount of extremely tiny particles which are removed with one's weekly water changes. What I can report is, in two tanks running HMF and a third tank which uses exclusively Poret foam cubes, I regularly vacuum a lot of debris from the bottom, especially in the tanks with wood, but the water itself is always crystal clear.
As somebody who has used a ton of floss in almost all my filters for years, it has been hard for me to give it up. In tanks where I am still relying on H.O.B.s or canisters with standard loading, I still use floss. But I have come to the conclusion that floss is more necessary the less bio-media one has. For H.O.B.s there is never enough media, so floss is the only way to deal with the dissolved solids. But this also means your filter is not as effective/efficient as a much larger HMF or similar volumes of high quality bio-media. I can also tell you that, when I do have to rinse the cubes they are really heavy, they are filled with organics. The same is not the case for hydro sponges or most other bio-media.
In a word, after one gains the advantage of closely controlled pores sizes combined with a rigid long lasting foam, by using enough of it one can handle most of the mechanical filtration needs biologically. There is no floss in nature, there is no weekly media rinsing in nature what there is is porosity and biology. There is no reason not to take advantage of this in tanks where we can.
The problems with most filters is that they are designed to do more than filtration and they are too small in terms of how much media they hold. If a filter must serve as a current/circulation creating device and an oxygenation device, then it cannot also be the optimal filtration design. The result is we must compensate by doing more work. We clean media more often and we swap out floss regularly.
While I do like the HMF and the principles behind it, not all tanks are suited for this form of filtration. This is especially true for smaller or display type tanks where there is not enough space.
While a well planted tank with a solidly established substrate offers great filtration capacity, such a design is basically useless in tanks when one has a bare bottom and/or no plants because they are not wanted or because they wont work. Of my 18 currently running tanks, only 3 contain lots of plants rooted in a deep substrate. A couple more have plants but mostly not in the substrate which is very shallow. I used to have many more, but the plants made a lot more work for me in terms of maint. They need pruning, fertilizing and created special problems of their own. Try catching a fish in a well planted tank, try even spotting them in some cases.
As for how much tank space I have lost using HMFs, I can offer these facts. In the 20 gal. long tank which is 30 inches long, I have a 2 inch thick Poret and use air power. So the space behind the foam is about 2 inches and I lose 4/30 or well under 15%. On a 48 inch 33 long tank I have a 3 inch thick foam and use a pump to power it. This takes up 6 inches, so I lose 6/48 or well under 15% again. My new experimental design using a pair of 2 inch thick foams at opposite ends of the tank and two pumps for powering them, will take up about 10 inches which is about 21%. But it gives me the flow I want to blow across cave mouths plus 1/3 more filtration capacity. I should mention that I make custom spraybar returns for the pumps rather than using a single stream. I am trying to maximize oxygenation for plecos this way.
Is it a good idea to use blocks behind them like small cubes in a random order or is that over kill? 
