Sorry to revive an old post, but wanted to say that it is indeed possible. In the first iteration of my 29G I had a few Otocinclus vittatus. I wasn't trying to breed them, but they somehow bred at least twice. The first time I almost did not notice it: I was about to clean my canister filter, and when I removed the media basket, I saw a single, tiny Oto fry swimming at the bottom. I don't know how long he'd been there or how he survived, but I guess by getting sucked into the filter he avoided becoming a meal for a larger fish (he was "stuck" in the empty space at the bottom of the filter between the intake tube and the sponge, so he thankfully never passed through the impeller). I put him in a QT for a short while to let him grow, but was concerned that the QT did not have any algae, so I soon put him in the main tank. He did quite well (there were lots of plants, so plenty of places for him to hide while he was growing).
After that first time I paid close attention each time I cleaned my filter, and a few months later it happened again, exactly the same way. As far as I know those were the only two times they bred in my tank, and both times only a single fry survived. Or maybe they bred a few times, but those two were the only times a fry was "lucky" to get sucked into the filter and avoid being eaten....
I wasn't trying to breed them, they just did, and I have absolutely no idea what I did to make that possible. But, it can indeed happen!
In case it matters, pH of my tap water is around 7.4, hardness is typically close to 100 ppm (about 6 gpg) and I had temperature set at 78. The tank was moderately planted with some driftwood, and I did 50% water changes every week. Other than that, I can't think of anything I did to prompt them to breed.
