Plecos are very "dirty" fish and will add a LOT of waste to the water. They also don't eat much algae. And most varieties of Plecostomus will get to be about 2 feet long, they grow fast, too......if you can return the Pleco, that would probably be best. The main cause of excessive algae is too much pollution in the habitat. This would be from the turtle and the Pleco making a lot of waste. Both turtles and Plecos are notorious poopers, LOL. The tub you haev is kind of small. Is it possible to upgrade to a larger Rubbermaid tote? A lot of aquatic turtle keepers use the big totes, I think they hold about 55 gallons or so. I believe they cost around $25.00-$30.00, though I've never really priced one. I don't know about the Algae Fix stuff, it might not be good for turtles. Here's a link to a good turtle forum, someone there should know:
http://www.turtleforum.com/