Why is my Turtle Tank getting dirty so fast?

I recently purchased a new 40 Gallon tank and a C-360 Marineland Canister Filter for my 8 month Mississippi Map Turtle. At the beginning there was 0 algae (which is good and bad for the my tank because i have guppies which feed off that and my yo-yo loche botia- sucker fish), this only lasted about 3 weeks before lots of algae started to form (on rocks, fake plants, and the basking docks). Im extremely surprised with my filter. After all, its the best filtration, is meant for a 100 Gallon tank, and has been recommended to me by lots of aquarium owners. I use a multi-purpose cleaner to remove all the chemicals. Before i bought my new tank, i had two 20 Gallon waterfall filters which kept my water insanely clean in the old 20 Gallon Long tank.. I positioned the new tank in front of my window so he can bask in the non-artificial sun. I think this may be my problem, I may be wrong but maybe the sun helps speed up the production of algae. whats wrong.. help!