I was able to have an FX6 + 1 AC110 in a 75gal for several years, both operating at full blast (i.e., not dialed down in any way) without having too much disturbing flow, and allowing delicate fish (guppys together with adult silver dollars) and plants. Note that 75g and 90g tanks are the same footprint. For the AC110, I used the empty soda bottle DIY tamer. I will not refer to it further today.
For the FX6, it simply came to the correct positioning of the outlet (picture below). Note that I could also used a spray bar (I have 2 other FX6 with spray bars), but in this case it was not necessary. Note that the spray bars sold on eBay for FX filters are NOT made by FLUVAL. They are generic but work perfectly, are good looking (black pvc), and are cheap!
Shown in the picture is the rear, right corner of the tank (for reference, the Intake is in the opposite corner, rear left). The two nozzles are positioned such that the RIGHT NOZZLE HITS the SIDE WALL AT AN ANGLE, so the flow slides forward along the side wall, then hitting the front and continuing circling the tank along the walls. The LEFT NOZZLE HITS THE BACK WALL also at at an angle, such that the waters hits the back and continues to move along the back wall. The overall effect produces a clearly visible circular motion, fairly gentle.
This worked for me very well for several years. Good luck!
