What I used:
Heres a pic next to my wallet for size comparison:
Now I cut the blue filter floss/foam off the top of the cartridge, and wedged in some of the prefilter in staggered order:
Then I grabbed a small coffee scooper and scooped some carbon into the cartridge. This is how water will flow through it:
And here it is in the filter:
And a bonus picture of the axolotl, he was actually standing.
So there's another crazy idea of mine. Now I have all three elements, mechanical(pre filter), chemical(carbon), and biological(bio wheel).




