I have found my oto's won't take algae tabs but then I have lots of algae so it hasn't been a concern. Now would be the time for you to try to get them to take a new food before the algae becomes a serious issue. Plus for plecos, they just need tons of food to thrive. I would try a combo of zuchinni or cucumber and algae tabs at lights out. Remove any uneaten food in the morning. It make take them some time to figure out its food so keep it up until they do.