Looking at you sig did you go and get one anyway? Otos like to be in groups of at least 3 and don't do well alone. They also need a bigger tank than 6 gallons as there is not enough room for algae growth in a tank so small.
Um just FYI on that....I inherited a 5 gallon that had been stuck under very close bright lighting for an entire winter because the previous owner didn't have a heater and used the light to heat the water.... I am STILL striving to get the lining of green algae off the walls of that thing....algae scrubbers don't work worth spit on it. Honestly whatever algae afix themselves to aquarium walls with, it should be sold as industrial strength bondo glue!
I'd love to give my ottos a go at eating that algae but the Betta who owns that 5 gallon tank wouldn't take kindly to them, besides striving to catch and net 4 ottos in a planted 30 gallon(their current home) is my idea of making greyhound racing look tame! So the ottos stay where they are and the algae is gradually coming off in it's own sweet time, when it ruddy well feels like it. ARGH.
If it's that hard to get off, it's probably green spot algae, and I don't think ottos eat that stuff. My rubbernose plec scrapes at it but can't really remove it. Is it a glass tank? If so, maybe you shoud try a new razor blade on it.