Thank you for the responses. I'm thinking of a 70 gallon and will probably put an FX7 or comparable on it. Clearly, I need to avoid running the co2 into the filter. BTW, you are correct regarding the fabricated throughput numbers: I recently saw this referenced in the small print in a mfg's literature -- they said the throughput was measured without any media or trays in the filter and with no tubing. Nice touch! The only good news is they probably all do it so you have an apples to apples situation when comparing filters. Thank you all again.