I'd highly suggest a cannister filter for turtle tanks. Submersible Fluval Filters cannot easily handle the bio-load that turtles, let alone fish, put on them. Also, I would suggest filtering for at least twice the water volume, so if you have a 75 gallon tank that is only half full, filter for 75 gallons. Or if you have a 100 gallon tank that has 60 gallons of water, filter for 120 gallons. It's a rough guideline, but will give better results in the end. Most turtle tanks are kept much more clean and bare than fishtanks for ease of maitenance and cleansliness- which means that the filter has more strain on it to keep healthy bacteria cultures.
That healthy invisible bacteria in the filter is what in the long run, keeps the tank water clean and healthy- which in turns means, clear and without stench. By taking your submerible fluval foam pads and washing them you are killing much of the bacteria off, making for an instable bacteria culture. In the long run, this means that the water will not be as clean, and that amonia spikes will be more liekly to happen.
Weekly waterchanges of no more than half the water, with adequate filtration would be the best route.
Plants can go a long way to keeping the tank healthy and clean too. Some that I would recomend are Egeria, elodea, anacharis, and if you have a fair amount of lighting; cambomba or sessiliflora.
Turtles like and benefit from having variety in their diet. However a lot of pellet food types/brands dissolve and sink very quickly which is a waste of food, and taxing on the filter and tank's health. My preferred choice of food is Tetra's ReptoMin Turtle sticks. The turtles love them, they float, dissolve slow, digest well, and when they are dissolved, they usually still float.