Can PitBull Plecos live outdoors?

All dogs CAN.

I wouldn't suggest it. They're tropical fish and any water temperature under 74 degrees F is not good for them. They're also messy messy fish and require some sort of filtration. You'd need to vaccum the barrel weekly to remove their waste/other garbage. Not to mention they have specific diet needs..as in algae wafers and veggies. They cannot survive on algae alone and might not even eat the kind you have growing.

Could you get a couple snails instead? They should eat the algae.

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I wouldn't suggest pitbulls. They are amazing little fish but they do need tropical, STABLE temperatures, which you just can't acheive outside with no heater. They also need a filter.
How bad is the algae problem? Nerite snails are pretty good algae eater, and an assassin snail would prevent the nerite population exploding.
What sort of algae is it? Hair or black beard algae can be spot-treated really effectively with Flourish Excel.