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I'm looking to get a 300 gal cow/horse through or stock tank to make a turtle pond out of.

What kind of filtration would that need? I see this thread but I'm lost: http://www.redearslider.com/forum/viewtopic.php?t=13255

And would raccoons & hawks mess with a 6" turtle?

Not sure with the filter, but it is absolutely possible for raccoons to mess with the turtles. After a week of putting our two little guys in our outdoor pond, we had raccoon prints all along the outer rim and we could tell they were diving down to get after the koi because of the plants getting torn apart. I would suggest putting some fence material that looks like eggcrate, but isn't (not sure what it's called; like a plastic fence), over the top of it.

You'd be looking for a pond filter. I don't have any experience with them, but SpotsMama and Terry does... hopefully they'll chip in

Im sure a 500 gal filter would suffice, id get a 100g capable one though and be happy. I did come up with a good home-made filter design today though.

I thought of having 2 2' long 4' PVC sections connected at the bottom by a single 1.5" tee leading to the pump. At the top you have cleanouts you use to clean the filter media etc etc. This can be sealed with an o-ring, cheap Thread tape, etc etc, Then you will drill a 1" hole near the top but on the side of each 4" pipe and JB-Weld or fasten with other means a 1" coupling in each section. join these sections with a Tee that runs to your fountain/fall etc. So it is kind of a hybrid freeflow filter mixed with a pressure filter.

I am going to construct one in the comming weeks, will post my experiences.