Mechanical Filter Versus Biological Filter??/

Well I have my Plexiglass top over my pond so the Raccoons dont bother
my fish/turtles ect. Half is plexiglass the other is green plastic
coated metal 2 inch by 2 inch.square so i think this year the raccoons
will stay away. I am not looseing my pets because of raccoons. Anyways,
I always used a basic plastic box filter with a course and fine filter.
Is that mechanical/ Or is it bio? The fish store told me it was
mechanical. Im totally lost here. I just want a combo bio/mech. That I
dont have to keep cleaning every week. At one time a friend gave me 2
350 magnum filters, they worked good but I had to replace the filters
every week. Any Ideas? 300 gallons, 4 hours sun, 3 turtles, 28 fish (to
many I know) Most 2-3 inches and frogs. Can i just use a 1200 gallon
pump with a diverter and run 1/2 fountain for air and 1/2 for water
movement? There must be a way to do this with the over crowdibg I have.
Im not adding ant thing else but I dont want to give up what i have.
Thank you