This is why I don't add a feeding tube.

Yes of course a remote controlled firmly sealing motorized door on the other end of the tube would solve this problem, but I know of no such kit and building such a thing from scratch is beyond my abilities. Besides which, food is not the limiting factor timewise, I can fit enough food in there to last for months. The limiting factor is how long I can leave it down before the litter needs changing, and if I need to bring it up once a month anyway I may as well use that opportunity to resupply food. Ergo, no feeding tube.

Airlocks and submarines

So, I have been prototyping different ideas for a safe airlock so that I can resupply each habitat with food without bringing it to the surface. It's a harder problem than it seems like. A moon pool won't suffice, since it lets in too much humidity over time and the hamsters quickly learn they can dive out of it. A double door airlock won't work because obviously they aren't smart enough to open the inner door by themselves and I can figure out no way of doing so from outside.

So, what I have come up with is a single door airlock/moon pool combo. When the door is closed, air coming down through the tunnel slowly forces water out the one-way valve in the floor of the airlock chamber until it is empty. When it is open, it floods the chamber only up to the top of the little doorway cut into the climbing tunnel, air continues to bubble out. I have researched relatively waterproof foods that come in large chunks that I can quickly put into the airlock, and found that glazed "treat sticks" fit the bill; Nuts, seeds and other goodies glued together with honey. Having submerged these and then allowed them to dry, they do not become soggy except after many submersions (the sugary outer layer eventually dissolves.) I think I could easily stick one of these into the airlock, close the door, purge the water, and it would be very edible after a minute or two.

I have also been thinking about Hamsub Mk2. The Mk1 was slow, clumsy, cramped and limited to shallow water. I would like the Mk2. to be fast, maneuverable, spacious and to carry it's own air supply. Towards that end, I was looking at the state of the art in RC subs.  As you can see, these days RC subs can be extremely fast, maneuverable and sophisticated, featuring everything from individually controllable pivoting gun turrets to battery propelled torpedos that really fire.

I have also discovered that the Playmobil toy line includes a number of surprisingly sophisticated toy submersibles with sealable, strongly watertight cabins, electric propulsion, and functioning ballast tanks. They also make a smaller but equally watertight "diving bell" that could be stripped down to the watertight portion and then added to a high end RC sub as the cockpit.

The specific sub I had my eye on for the basis of Hamsub Mk2. can be seen here. It's very robust and already ballasted appropriately such that adding the modified toy diving bell to the front would not affect the balance (in place of the viewing bubble, which does not appear to be water tight.) For the onboard air supply, I would sling one of these micro scuba air tanks under the sub's body. I'd then affix a valve that would let me set it to very slowly release the contents into the 'crew cabin', which would vent bubbles continuously out a one way valve in the floor. Because both the air canister and the proposed 'crew cabin' come pre-ballasted to neutral buoyancy, neither would upset the balance of the sub itself. Based on the rate at which a single hamster consumes air, that canister could easily provide over an hour of life support between refills. As it's still an ambient pressure design, the safe depth would be no deeper than 21 feet, although that's ample considering the city itself will be no deeper than 8 feet.

P.S. big thanks to Scroton, who recently donated a 50 watt solar panel and additional battery pack; With these, it is possible to keep a colony running perpetually even far from any source of power, by charging one battery while the other is in use, and periodically swapping them. The solar panel will ensure a single battery lasts around two days on a charge and the air pump has it's own integrated battery so it will keep running during the swap.