On the recommendation of a clever follower of the blog, I've researched PVC valves, flanges, fittings and flexible duct hosing with which to safely connect multiple habitats while they're submerged. Before, I had imagined a ridiculously overcomplicated system of wirelessly signaling arduinos to open interior O-ring sealed hatches using servos in order to safely "dock" modules to each other while underwater. Way too expensive and fault prone.
The only alternative to this I could see was to permanently mount every enclosure you wanted to be connected to the same rigid, custom milled metal platform so that when you join them with rigid acrylic tubes, there's no stress on the spots where the tubes pass through the hab walls. But this has to be built as one piece, cannot be expanded afterward and would be tremendously heavy. Given that the whole thing would need to be removed from the water every time you want to clean the litter or resupply food and water, this approach is no more feasible than the last.
So, along comes this guy to suggest a better method which is honestly as big of a breakthrough as when I found the ideal ballast pods/weights combo a while back. It will make it possible and relatively easy to join as many habitat enclosures as I have money for, and to resupply food and clean litter without surfacing all of it.
Basically, each enclosure will have a flange mounted to the wall, and a gate valve mounted to that. They are like little watertight doors I can open and close from the outside, by hand, by turning the little knob. The idea is there's one of these on each of the four walls so you can expand from it. The way you expand is by sinking another habitat next to it, screwing one end of the duct hose onto the gate valve (which will need a threaded screw-on fitting for this) then screw the other end of the hose to the gate valve on the other habitat. Then you use a bottle of compressed air to purge the water from the hose interior through a little, sealable opening in the lowest point of the middle of the hose. Once the water's been blown out in this way you can seal both the bottom opening and the little fitting you blew compressed air in through, open both gate valves, and presto the habitats are connected. If this sounds hard to understand, don't worry, at some point I'll do a step by step illustration. Rest assured it's entirely feasible.
How the food resupply/litter replacement works is that the smallest module size (transparent Otterbox 3500 bolted to a ballast pod consisting of a black otterbox 3000 with two 5lb diving weights inside) is used as a swappable food container, and another identical to it but with only litter inside is the "bathroom". Hamsters typically pick one spot to piss and shit, and you can control where that is by putting some of their dirty litter in the spot you'd like them to do their business. So you do that with one of the small modules. Mostly clean litter with a small amount of waste to clue them in to the fact that you want them to use it as a bathroom. When it gets foul enough, you close the gate valves, detach it, swim to the surface with it, and empty it into the trash (keeping a small amount of the dirty litter.) Then you put fresh litter in there, and add the small amount of dirty litter to it. Now you can swim back down to the colony and re-attach this module to it. Presto! Same goes for the food module. Resupplying food and removing waste is now, finally, feasible in a low tech and easily built method. This means I can finally have very large networks of habitats that permanently stay underwater and never need to be surfaced! Incidentally, a small module of the same size as the food and waste modules would be used to transport additional hamsters down to the colony.
The water bottle replacement scheme is even simpler. The hole in the roof that the metal drinking straw/tube passes through is now covered by, for lack of a better term, a stretchy rubber sphincter. The idea is that you can push the water bottle's tube through it and it forms a seal. But also, when you remove the water bottle, the hole in the rubber is tiny enough that bubbles escape but water doesn't enter. You can just quickly swap the empty water bottle for a filled one without ever risking any amount of water getting in.
For the first time, every conceptual piece is in place for a truly permanent underwater colony of any size I'm able to afford. That was never the case before, not the way I thought I'd have to do it. Now, provided the funding, I can easily supply things people have been eagerly asking for: More living space in the form of multiple enclosures linked by tunnels, true permanence to the point that I never have to surface the habitats (except small food/waste modules) once they're in place.
So, which specific parts and why? I've settled on a two inch diameter standard for the hose, valves and fittings so the hamsters can squeeze by one another (Their bodies are about an inch wide). Otherwise even in very short tubes they will sometimes both try to pass through in opposite directions and fight over who gets to pass, as seen in some of the early videos with Hampture Mk.2.5. They are pretty much the Zax, in rodent form. Anyway here's the parts list I've come up with. Won't let me link directly to the specific parts for the first and third items, so I've included the model numers/names for you to plug into ctrl+f.