Thank you for this tutorial. I bought a 20 gallon long and wanted to divide it into three sections for my bettas. I only have one problem -- I used #7 black plastic mesh from Michael's craft store and the holes are big enough that the fish can see through it. I think it's causing a problem with one of my males because he has now gone psychotic and chewed off his whole tail.
The mesh I found is not very flexible. I notice the one in the first post came in a roll. Is this sold at a place like Michael's as well? For those who have used it, can your fish see each other, and has it caused a problem? I tried adding some tall silk plants on each side of the dividers to break up their line of sight, but they still find each other and flare.
I also had to get a bit creative with the edges where the binder spines meet the tank glass. I found that if my fish pushed just right, they could actually move the divider. I cut a 2" strip of the mesh, put a binder spine on each side of it, and shoved it between the divider and the glass so that it forms a T. See my picture example:
It also seems to help stabilize the dividers. I sewed these ends onto the main divider so that it all moves as one unit and I can easily remove them for cleaning.
FYI, I also put a panel in the back of the middle compartment to hide the heater and filter, and the mesh also acts as a baffle of sorts to break up the output flow from the filter. Multitasking. :D