Oto's are very small fish and so have very small guts. The smaller the gut, the less well the fish can cope with going without food for long periods of time. From the fish breeder to the petshop Oto's tend to suffer a lot, and by the time they arrive at the petshops tanks they are stressed & hungry. A lot of petshops don't feed their fish enough food and so the oto's, in a shiny clean pet shop tank clear of algae, then loose more weight.
By the time they are bagged and bought by someone, they can be very thin (all oto's i've seen or bought from pet shops have been underfed). In a stressed & weakened state they are also vulnerable to picking up internal infections (bacterial or parasitic).
While completely peaceful by nature they are also very shy fish that can lack confidence sometimes in big community tanks (the bigger the group of oto's the more confidence they have). If the new oto is stressed & weak, if put in a tank too boisterous even when offered food the oto might not have the confidence to go an eat it. So the key factors in successfully establishing oto's in your tank are;
1. Buying the fish: Pay close attention to the fishes body weight & colour. A concaved stomach equals either a very underfed fish or a fish possibly suffering from internal infection. Colour: if the oto is pale and grey then it is either very stressed or dying or both. Choose the fattest and least pale oto's as these ones have the highest chance of settling in well into your tank
.
2. Acclimatisation & tank habitat: Acclimatise the oto's very gently to the tank with the tank lights off. Ensure the tank has plenty of planting as oto's enjoy planting in tanks and spend a lot of time in planted area's (my oto's in particular love a particular fake plastic plant in their tank and spend a lot of time either perched on its leaves or cleaning it). Being in a suitable habitat will reduce their stress levels and so improve their chances of successfully settling in.
3. Tank mates: Oto's do best in peaceful tanks that aren't full of aggressive, territorial or boisterous fish. Bottom dwelling fish like Clown Loaches & Yoyo Loaches might be peaceful but they are rather boisterous fish which have gluttonous apetites and might not let the Oto's get a chance to eat any algae wafers put into the tank for them. More chilled out bottom dwelling fish like Cory's, Khuli Loaches & Pleco's etc make better bottom dwelling tank mates. While Oto's are not bottom dwelling fish, they don't eat fish flakes but rather the focus of their diet should be algae wafers which sink to the bottom of the tank (they do eat other foods too though they're more or less algae eating fish). So what bottom dwelling fish the oto's may share their feeding times with is an important factor.
If you have algae like Brown Algae then Oto's love this stuff and will quickly fatten up on it. Getting your Oto feeding is crucial to its chances of successfully settling down in the tank. The quicker they put on weight the better their chances of surviving.
Summery: So basically, Are Oto's sensitive to water quality? Yes & no. A newly bought oto which is stressed and underfed is much more sensitive to outside stresses like stress from water quality problems than an oto which is well fed and settled down. In fact once they've fattened up and settled down they tend to be quite robust little which don't require anything other than a suitable habitat, good food and stable water quality. If you add oto's which are stressed & underfed to a tank that has water quality problems then this is often all it takes to finish them off, but as long as you go about things the right way with them then they're very good problem-free little community fish
.
). Seeing poop in the bag when you get them home is a great sign, means they're eating. 
