thinking of getting a canister filter for my new 120G that i am buying on black friday. I am debating wheather to get one that is rated for 400G or the one that is rated for 100G or maybe even 70G. as far as I know, the only difference is really the flow rate. the higher the flow rate the more expensive the filter is. could it be just a marketing ploy? do we really need that high flow filter?