Well, it's been over a month since I updated this journal, but I do have a big update.
Just finished moving house, including the fish. Genuinely one of the more stressful experiences of my life, and I didn't even have to drive anything (just moving to different on-site housing (for stupid administrative reasons) so it was within walking distance from my old place). Not to mention, physically exhausting. The fish seem to be doing alright so far, but I'll be keeping a close eye on them and testing water regularly for the next week or two.
I did make some slight changes. They're in the 20g long now instead of the 25gal. I'm going to be repurposing that for a tree frog vivarium, I think. Plus, it was easier to set everything up in a tank that was already empty. I brought over everything from the old setup, but left a solid 1/2 of the substrate volume behind. I realized there was just way more substrate than I needed in the old tank. When I started taking substrate out, I discovered a rather large patch of anoxic bacteria. Well, obviously it hadn't been harming the fish (and they were all out of the tank by the time I stirred up the patch), but I figured it would be better to have a shallower substrate moving forward.
You may also notice that the tank is on a stand! If you can't tell just by looking at it, it's homemade. It's just 2x4s screwed together with wood glue, stained, and sealed with a water-based polyurethane. It's not particularly beautiful and nowhere near perfect, but I'm overall happy with it. The 25gal will eventually go on top. I'll definitely have to be a bit more careful with water changes moving forward since the polyurethane isn't fully waterproof, but that's easy enough.
The only other "changes" to the tank are the plans for it moving forward. I think I'm going to give up on the hatchetfish... There's an LFS that tries to keep them in stock, but they seem to sell out quickly and the supply isn't constant. They apparently never know if they're even getting hatchetfish until the fish shipment actually arrives, so I can't even call ahead and plan. They're nearly a 3 hour drive away from me, so that, combined with the stocking uncertainty, means I just can't justify keeping them in my stocking plans. Additionally, the logistics of a screen top are just not looking as feasible. It'll be better to keep the tank at a lower stocking level anyway.
Lastly, I'm considering breaking the Ucayali biotope almost completely and getting a bolivian ram instead of an apisto. I'm not sure on that decision yet, but considering how gross and stagnant the sand gets sometimes (even with weekly substrate vacuuming), it would be really nice to have a fish that regularly sifts through it, even if it's not actually eating the detritus. It might help prevent the anoxic pockets and just keep things looking a bit nicer.