As per the good advice you have already received, I can add my own experience.
I have 2 tanks that use FX6 filters (together with other filters). Definitely it is good to have more than one filter in largish tanks, although I do it in all my tanks-
1- 75g with FX6 + AQ 110 (picture below). FX6 intake on left corner, standard 2-spout outlet directed so than one outlet hits the back wall and flows lengthwise along the back of tank from right to left. This produces and overall surface flow.
AQ on right corner (but due to AQ configuration, the intake is somewhat away fro the corner.
2- 125g with FX6 + AQ110 + Penguin 350B (second picture). FX6 (with spray bar covering all of central part of tank and a bit of both sides of tank), and intake on left of tank. AC110 on left of tank, Penguin on right of tank.
Both tanks have been set up in this way for quite a while and I have been happy with the results.
Good luck!
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