I'm not sure what the total amount of water would be once it's done. It would depend on how I set up the divider and the depth that I fill it to.
The link that you posted basically has what I wanted, except I was thinking of a higher divider than the 4" posted there. I was thinking I could use acrylic, because it's more flexible and easier to work with (ie if it's cut a little too big, I can just trim it down, or bend it to fit).
I'm not sure I understand how a sloping tank would work, in terms of keeping a Betta in there. By the time the slope got to the other side of the tank (in order to keep the slant such that it didn't all slide to the bottom of the tank), there wouldn't be any space above water left for the plants. I like the idea of having water plants in there, and will likely find some bottom-growing water plants to put on the Betta side of the tank regardless of whether I fully divide or slope the tank.
Oh sure, they had to go and mention a tank that had a waterfall.. now I can picture me trying to build a filter-system that runs the water down a mini-stream where the plants are and then back into the fish part of the tank. My imagination isn't good for my wallet. I think I'll need a much bigger tank to achieve what I want, and I don't have space in my house for anything bigger. Grrr.
My bf had a Betta previously, in a very small (1 gallon maybe?) vase-type container. He changed the water once a month, completely. Fish seemed happy and lived over 2 years, which I know isn't super-long, but I understand that's about average. I don't mind changing the water, I just need to know how I'd do it in a tank that I can't manipulate. Do I just suck/siphon out some of the water and put clean water in? Or would I have to put the fish in something else and then siphon out all the water and change it? What's the point of filters if you still have to change the water completely?
Thanks for the link and your help so far.
Edited to add; I found this link for a build-it-yourself viquarium (really what I'm looking for). I figure with a few modifications, this concept will work for me! http://winkrdink.tripod.com/index.html