Well 24 hours later, it seems some of the "pecking order" has changed within the tetras, and the territorial zones are different from yesterday. Different zones seem to be contested over at different times, and cories tend to be harrassed if they wander into the "brawl zone" unawares. I just watched as a single tetra was trying to mark its territory in the front lower left of the tank, only to have a group of 4 cories or so sniffle about, the lone tetra doing its best to take little nips and charges at the cories in an attempt to get them to clear out. The cories, it seems, are too stubborn or too dumb to take the hint, and even after being nipped at, would quickly return to the site to join its friends in sniffing the sand.
More observations: feeding is turning out to be a chaotic affair as well, because they tetras have no qualms about feeding from the substrate. After already feeding them with flake, they will then seek out the area where the cories are enjoying their sinking tablets or shrimp pellets or whatever, and literally take chunks with their pointy-toothed mouth and simply run away with it! I will have to probably start feeding the cories at a later time, perhaps at lights out time when the tetras are less active.
Byron, as you suggested I put a piece of sponge between the glass and the wood to create a small tunnel, only about 1.5 cm in height perhaps, and amazingly shortly after i set it up, the TETRAS are swimming in en masse, maybe a group of 3-4, going in to check out the new changes, and then what ensues is something resembling a knife fight in a phone booth! Apart from the smallest of caves, they will literally enter any crevice, and then brawl in it. Utter complete savages, these fish.
Oh, one piece of optimistic news in view of the cories is that they seem to be spawning and laying eggs on a daily basis. I wasn't sure what the egg-like stuff was, but I confirmed it once by watching the mating ritual happen before my eyes, and the mystery white stringy stuff deposited on a Pygmy chain sword
leaf. I'm finding new eggs every day, but I assume unless I purposely remove them none will survive - they're not laying enough en masse, and most of the eggs seem to fall off the leaves onto the substrate where they're probably consumed or destroyed.