Is this a planted tank? What type of algae do you have? Otos only really eat diatoms.
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^ This. If you have hard green spot algae or black beard algae, they won't touch it. They only like the soft diatoms and hair algae since their mouths aren't strong enough for tougher stuff. They also will do nothing for green water. Stick to reducing your lighting period, increasing water changes, and ensuring that you're not overfeeding if you want to limit the new algae growth.