Best filter setup for a 75G with a slider/pleco/comet

I know I need a larger tank for my poop machines -- an adult Yellow Bellied Slider, large hi-fin pleco and a comet -- they've grown up together in a 29g and I feel bad that I haven't been able to afford a larger tank before now. I can accommodate a 75 gallon in my home, but what's the best filtration? I have an Emperor straining under the weight of filtering the poo production. I was thinking of adding a canister filter such as the Rena Filster to deal with the mechanical filtration, but will the Emperor be enough for the biological needs once I've gone into a large tank? Many thanks!

Thanks -- I agree that they need double filtration. Yes, the filter's an Emperor 400. What are the best canister filters? Thanks
I know in theory the turtle could could at least injure if not kill either the comet or the pleco, but they've been together for over a year and seem to have reached a good symbiotic relationship. The turtle only gets annoyed when the pleco tries to attach to his shell. Looks like Gamera vs. Rodan in very slow motion.
Thanks much for the info!