Planted tank? If so lightly or densely planted?
Otocinclus are a scaleless armored catfish in essence they have bony plates and not scales so they do look different. Also - I personally think it's difficult to figure out if they have dropsy. I have a two year old school in my 29 and when their bellies are full, they do get pretty round. Incidentally, my pH is around 7.8-8 (Boulder water) and they do fine.
Otos are generally a wild caught fish, so they're initially a lot more fragile. This is a fish that should really be purchased from a good LFS or hobbyist as you will get healthier fish less likely to die on you. Once they've adjusted to a healthy tank, though, they're usually not too touchy, though they will react faster than most other fish to problems in the water. Since it seems like your otos had been in the tank for several months with no problems, my guess is that the problem originates with some unusual change in water quality.
Now I've been told by some of the guys that live in S. Denver that their tap water gets ammonia spikes. One guy told me he lost thousands of dollars in fish as a result - I think he was breeding Corys and Ancistrus if I recall correctly. Anyway, with your mentioning of such large water changes, if the tap water had an ammonia spike and you did a large water change... well you see where I'm going with this. You might want to keep your water changes down to 20%, which is standard for most folks. I would also recommend that given this particular issue in the Denver area, it might be a good idea to test your tap water before adding it so you know what's happening and can take appropriate measures if needed.
This also brings me back to the plants. Many plants will actually uptake ammonia directly before it even has a chance to go through the N cycle and become nitrate. As a result, keeping plants in the aquarium can provide some protection with a situation like this, assuming ammonia in the tap water is the cause. Hence my question about whether your tank is densely planted.
You also might want to try coming to one of the Colorado Aquarium Society meetings. You don't have to be a member to attend, and there's a lot of folks there from the S. Denver area who have tremendous knowledge and experience, and they're all pretty friendly and welcoming.