Yours is a tall order to fill. Since daimondbacks are almost as incapable as some of the Graptemys at dealing with high nitrogenous waste levels in their water, you'll have to strike up a strong combo filter that will address not only the large amount of solid waste entering the setup, but the invisible waste as well. This is particularly true since nitrogenous waste is more abrasive to life in salt water than in fresh.
An aquarium canister filter (i.e., the "affordable" ones) is likely not a viable option. The largest Filstar is a strong filter with a fair amount of media space that provides very good accessibility, but you'd likely need two or more of them to do an ample job on either the tank or the baby pool. If you can remain diligent about maintenance of the filters and media, this is a good candidate, and the model (XP3, I believe) can be ordered online for but a hundred bucks. Considering that a good pond filter would cost you more than two of those Filstars, not a bad possibility.
Mechanical filtration will be the first and most visibly necessary step. Find a filter with a good foam block. And do remember, you'll likely have to clean the block at least a couple of times a week, so get one that has a maintenance-friendly block. Filters don't actually clean all of that visible waste out of the water. They simply catch it and retain it for us to remove, and that doesn't happen till we pull out the block and scrub.
Second is chemical filtration. That's going to target nitrogenous waste and many other impurities which abound in any turtle water. Get a filter with an easily accessible media basket and plan on spending some money on carbon. This step is often skirted but I've found it to be very important in keeping the water "clear" by way of a filter. Who would want to stare into 175 gallons of fairly-clear water? The "research grade" carbon by Aquarium Pharmeceuticals is very good stuff, and you can get a huge amount of it for a little over a hundred bucks (that includes shipping) from drsfostersmith. I've had better luck with longevity in that stuff than with other easily-bought high grade aquarium carbon types such as black daimond (Marineland), which is a fair product in its own right.
You can skip the second step if you provide enough of a media vat to support full biological filtration, which is the third step. Bio filtration breaks down the organic nitrogenous waste that you don't want in the water, but it won't pull out inorganic molecules as will chemical filtration.
Insofar as choosing among brands or models, there are just too many. I quit using any of them as, electrically, they are inefficient for most of my applications. But, separate-pump-driven outdoor pond filters might be what you're after, since they're very big and thus hold lots of the media you'll need. If you decide you don't need anything that gargantuan, the Filstar might be the canister for you. They're sturdy, don't clog easily, are readily accessed (snap-latch top with three separate media baskets that rest atop one another), and run fairly quietly.
It's been some time since I revisited the comparison pages at the following site, but, since much of the discussion is brand-specific, it might be of help to you (since the discussion pertains directly to turtle tank application). Austinsturtlepage.com is where you will find such a topic addressed. Then again, you also might surf the web and see about designing a filter setup for yourself, tailor-made for your own application.
