Why has my fishtank filter stopped working right?

about a month ago I bought an "Internal power filter" by Tetra, it worked perfectly when i first bought it but within the past few days it still puts clean water out but the flow isn't as strong as it used to be and so this slimey s forms on the top of the water, and the water isn't as clean as it used to be. I changed the filter to see if that was the problem but even with a new filter, rinsed with cool water just as the directions read, it still isn't working right and the water gets dirtier and dirtier as the days go by, it's for a 10 gallon tank "for 2-10 gallon aquariums" why isn't it working?