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Suggest you respectfully ask the salesperson to feed the fish so you can see it eat. If they tell you "Sorry, it has just been fed!", go out and find yourself a more reliable retailer.
Sorry, I was being facetious (holy cr@p! I spelled that word correctly the first time!) about the 300G tank. I actually have no idea what size tank the poster intended to put these fish in. I think putting both in anything less than a 180 is just an accident waiting to happen...
They should both be OK in a 300 Gallon tank. That said, Achilles are notoriously difficult to keep alive, and Powder Blues are Ich Magnets, especially when they are stressed.
When one makes a hole in one's outside wall for a pipe that will feed outside air into a skimmer, what do you use to prevent bugs and other critters from entering that pipe? Won't some sort of filter obstruct the flow of air?