Welcome back, Snaps!
Good point about territories, I didn't think about the sand that way before. . . I like the idea of separating the edges
out - and you know I like my slopey sandbed. Hmmm. . .
For now I'm still going with the theory that the sands will
inevitably mix. Even if I had an effective barrier method, the my beautiful little earth eaters will see to it :) and I'll be really sad to have sand speckled light and dark. I really do wish I could find some shades that would blend well - I feel like I could do so much more if my options weren't so starkly opposite - black and nearly white. I also intend to keep the overall tone of the substrate on the darker side, as I feel it lends the fish confidence and strongly influences coloration - most especially in the rams. I'd love to encourage a 'natural' mixing between two or more areas, but it would have to be sands that would also look good (to me) when mixed up entirely - and I haven't been able to find anything that would work with that in mind.
I do create 'natural' territories for the rams inside the 55 already using rocks, wood, plants - and even the leaves. And for the loaches, I have added in an area where the sand bed drops away and the older leaves are left to disintegrate. It's impossible to get a photo of, because it's behind and slightly beneath the large rock to the right of the tank, also as they prefer - but I can see it in person, easily reach it for cleaning, and they really seem to be enjoying it!
Along these same lines, I've gained enough of an understanding of all of the fish I have to be able to pick out some distinct preferences, and intend to use what I've learned from them to the best of my ability to scatter little 'havens' for each type of fish throughout the larger tank. For example, the Rams are very visual with their territories, and will always choose an area that is guarded on as many sides as possible (including above) by plants, driftwood, and rocks - but with a sandy 'floor', while the loaches like to feel that they're tripping through a forest, with plants low to the ground or layered, 'natural' dips in the substrate, gaps between stones, and leaf litter (hoping I'll be able to do something wonderful for them in the larger tank with the dwarf Sag that Termie is going to send!). The Tetra prefer an open area for swimming, uncrossed by driftwood or plants, and with the feel of a solid bank behind them to lend security.
As new creatures are added to the larger tank, I'll be making changes to the 'aquascape' with their preferences in mind as I come to understand them (betting the gourami are going to teach me which floating plants are the 'right' ones). I'm really looking forward to getting something in the 125 that is ideally suited for those who will be living there, as well as being aesthetically pleasing to me.