You're right, you do need something to hold the stack down. After experimenting a little, I devised a "holder." Heres how to make it:
You will need:
1 - 1/4-20 Nylon Threaded Rod
2 - 1/4-20 Rod Coupling Nuts
2 - Comfort Grip Crochet Hook Covers
Cut the Nylon Threaded Rod into a piece the *just* barely doesn't fit diagonally from one corner of the Aquaclear filter chamber to the other. Now cut the piece in two off center, there should be a little piece and a big piece. Screw one piece halfway into one end of a Rod Coupling Nut, then the other piece into the other end. Pick out the two large Crochet Hook Covers from the pack, and insert each hook cover on each end of the Threaded Rod / Coupling Nut assembly. The resulting piece should look like this:
Make another one just like this. Insert them on top of your Aquaclear Media stack to form a "X." Now, on each of these assemblies, you can turn one end and tighten it! The Nylon Rod is also a little flexible so it
will bend instead of your Aquaclear warping. The Crochet Hook Covers hold the Rods in place.
You can if you want put a little adhesive
on one end of each of the assemblies and screw the adhesive end only a little bit into the Coupling Nut. Let dry before using, and this will give you more "tightening" length and make it easier to turn and tighten.
Here's my custom Aquaclear 300 Stack, from bottom to top:
1) About 1/2 cup Eheim Ehfimech
, enough to make it about 2 "rings" deep.
2) Aquaclear 300 foam.
3) 1-1/3 cups Eheim Ehfisubstrat Pro
in a 6x10 800 micron bag
4) 1-1/3 cups Seachem Purigen
in Seachem's The Bag
5) The Threaded Rod assemblies holding it down.
In addition to this, I wrap a Fluval 104 Foam Block
around the intake of the Aquaclear 300. I hold it in place with two rubber bands. This works as a great prefilter. Clean it every day, and it will keep the flow rate up and the dirt accumulation rate down inside the Aquaclear. It also is a safety measure for a fish getting stuck in the Aquaclear's intake suction.
This all sounds extremely complicated, however it really isn't. Once you put it together, it is surprisingly simple. Seachem Purigen is really an amazing thing to behold. This will make your tank look like there's no water in it and the fish are floating. It also lasts a lot longer than carbon, and doesn't absorb trace elements like carbon does.