Why wont my turtle tank stay clean??!!?

Ok, I have a res turtle aprox. 4 & 1/2 in, he is in a 60 gal tank with a big floating bask area with a full spectrum 100 wat bulb I have 2 fish in there (were originally feeder fish but I guess he likes em too much to eat them lol) one is 1 in and the other is 3 in, they eat off the bottom like catfish so I don't feed them any fish food. I have about 2 in of rocks (big enough that he can't eat them but not huge) and 4 fake plants. I have an aqueon power filter 75gal and a tank heater that keeps the temp at 78-80 deg. Ok so now that I'm done with the spects ill tell you how I clean it, I do a vacuum of the rocks once every two weeks and change out the filters (my filter holds two cartridges) every week and put in a biological turtle waste and odor control at the same time, I also to 25% water changes every week, so why is my turtle tank always dirty??? It always has alge (I tried to get pleco and alge eaters, my turtle just eats them before they can make a difference in the tank) and it always has these like green thick but like half dime size spider web looking stuff floating around and the filters are sooooo dirty by the weeks end, I've had the filter for like 2 months and I had to clean out the whole thing because it was full of alge stuff.... am I doing something wrong??? I know I'm not supposed to feed him in his tank but he won't eat unless the water is over his head and I don't have a tub that big to feed him in other than his tank. Plus I don't want to have to empty a huge plastic tub twice a day and wash it out and fill it up twice a day.... pls no smart @$$ remarks, I am trying my best and doing the best to my knowledge and those kinds of answers don't help anyone and just make YOU look stupid because ur the one who wasted ur time reading my whole post just to say something dumb. Thank you for the ppl who read my post to actually give advice, I appreciate it. :)