I've kept mine in small a large tanks. No oto survived long term in a small tank. All ottos I kept in my 100G survived for years and were still alive when it leaked.
Otos are great swimmers. You will only notice that if they're given the space. They'd fly like rockets around the tank and they can in fact be very active. The tank I kept them in was in the sitting room and every evening after tank lights out they used to "dance". They'd be going in circles around the glass as if they're chasing their tales, swimming rapidly from side to side, playing in the water flow, all kind of weird stuff I never saw while I kept them in small tanks. In a small tank all you'll see is them grazing on something, and they'd be rather stationary.
Compared to other algae eaters, have you noticed the difference in their body shape? It is narrow and streamline, like in all fast swimming fish. The only reason they're recommended for small tanks is because they're small fish. Its just marketing. Also, otos are not exactly algae eaters. They eat tiny micro-organisms growing in the algae and biofilm. For example their favourite is diatoms, which also isn't algae, but micro-organisms type of brown film.
Having said that, with some good care you can still keep them in a small tank but there's a chance they won't be very long lived in there. The minimum I'd recommend is just 2-3 ottos in a 20G at least.
Sorry to hear the thin otos are dying. Its happened to me at purchase, some don't make it. Its the consequences of transport, stress, # lack of food leading to loss of gut bacteria, or how they're caught, etc....
But It is a good sign that at least some are eating and getting fuller. If you can't find flubendazole, I'd recommend you don't dose anything at all and wait and see. Many meds are toxic to otos. The thin ones won't make it. Its a matter of time for them to die. Concentrate on the ones that are eating. Do plenty of water changes to help the fish get strong. It is really important when purchasing any fish, to do large, even daily water changes.
