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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?