I was told on *Australian Freshwater Turtles Forum* by a very experienced breeder and also the creator of the forum that it was 4x minimum. Obviously bigger the better. The smaller your filter the more maintenance you will need to do on your tank.
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I keep 2 Britlenose Catfish in with my turtle to keep the glass clean and they definitely do the job!
Have you thought of breeding guppies? Cheap food for your turtle and if they last long enough will make your turtles tank look nicer as they have some colour to them as well
I buy neon tetras and throw them in my turtles tank every now and then, and breed guppies. I find this method works the best for me, and is also cheap. DO NOT USE GOLDFISH OR ANYTHING SIMILAR they can cause fatty liver and that is definitely not good.
Good luck
Tyler