actually the idea of the HMF is, that you never have to clean it. At least that's the theory...
I will be away for a week now, so I may be slow to respond.
I'm one of the critical folks about them. All the reviews I've read on them say something like "you never have to clean them until you have to clean them sometimes."
My only real problem with the HMF is that people claim it's something other than a large sponge filter. We all know that sponge filters have some great benefits. They are one of the most effective biological filters, second only to a wet/dry, IMO.
I think I'm going to do an HMF on my 15 gallon, but I refuse to call it an HMF. It will be a VLSF. A very large sponge filter.