I too have an FX6 on a standard 75gal, along with an Aquaclear 110. Sufficient but not overkill, and not too much flow if you set it up correctly for your needs.
Mine is a planted tank, see below how it looked a couple of months ago.
I also just set up a 125gal (no fish yet), with a FX6 + AQ110 + Penguin 350. It will receive the inhabitants of another 75gal that is being re-assembled.
You can also trim the intake tube to make it shorter and if you save the cut piece you can use some hose to reattach it in the future if you get a taller tank.
Is it ok if i put the intake stem strainer horizontal
Cos my tank 1.5ft high..with the strainer so long, only left few cm for substrates
I reckon can only put 1cm sand
I do not wish to cut the stem, to keep it as original
Or i have to put some filter wool inside the strainer to prevent sand from goin in