Planted Tank Enthusiast
Join Date: Dec 2008
Location: Kansas City, KS
HMF is a VERY different beast to a sponge filter. Especially the "sponge" filters we Americans pimp. Most sponge filters try to get all the water they can through the sponge to allow the MOST biofiltration possible. Sadly, this is completely backwards from the actual truth. The contact time required for bacteria to do their job, and the layer of mulm required for the HMF to do its job is just not possible with sponge filters. Even the air driven sponges have too much flow through them. The surface area of the HMF is what allows you to have a 2x turnover rate through the sponge while still allowing the water to pass through any given area of the sponge VERY slowly, allowing the bacteria to actually do their job.
You can get a LOT more technical than this about it, but I won't because I can't even remember the math behind it (it's made by German engineers, so there's a LOT of math) but the bad part about it is that in a heavily planted tank, it's just not that needed.
What it WON"T do (that sets it apart from sponge filters) is mechanical filtration. Sponges are the BEST at this job, but not so much in the biological catagory. (especially powerhead driven sponges) The HMF is just a terrible mechanical filter, which also makes it a fairly bad candidate (or at least not optimal) for most finicky planted tank owners. (myself included)
Anyone who has had a real, and by real I mean built with the correct math in mind, HMF in a tank for any amount of time will tell you that they do NOT need cleaning. By cleaning, I mean removing to squeeze out, as you would a sponge. If you think the mulm is unsightly, you can simply run your python over the outer edge of it to pull any matter you feel doesn't LOOK good to you, but it won't harm the filter or its capacity one way or the other. Most people simply turn the MHF into a moss wall and it'll still do its job just as well with the moss hiding the works. Great place to hide heaters, etc as well.
Keep this one updated. I'm anxious to see your design in action, as I've never seen one that split the sponge like that to hide the ugly seems. (well the seams would be ugly if I made it anyway lol)
*edit* if you make it an "American version" of a mattenfilter, like the WasserMattenFilter or some such (no offense to the DIY guru himself I love your plywood tanks lol) meaning you up the flow to accomodate the mechanical filtration needed, then yes. you've just made a big sponge filter. I'm talking about REAL mattenfilters designed and implemented as they were intended.