It would give u more flow/turn over. But...it's going to take up space in ur tank, it's less versatile in directional flow, and I'm not too sure about the effectiveness of filtration. If ur going to use a secondary filter, I would go with a good hob
filter. I prefer (as I'm sure u realize) the aqueon 55/75. If it's positioned opposite of the return of ur other filter, that should take care of ur surface junk and shouldn't hurt the plants. But a powerhead or two won't hurt. 2 biocube 250's would be a lot more versatile than the f u4 and shouldn't create too much flow. And I always use 2 filters that are rated for my tank size at at minimum. The 55/75 is rated at 400 gph
and for a 90 gallon tank. I have a 75 gallon so I'm running a manufacturers rating of ~ 800 gph
and a manu of 180 gallon tank. Then a total of 1150 or 1200 gph
powerhead flow. So I'm rated at a combined total of 1950-2000 gph
of turnover. In a plant less tank u look for 10 to 20 times an hour turn over. I'm standing at about 16-16.66 times an hour turn over. I think. M doing long division and it's 0417 in the morning where I'm at and since ur measuring in liters, I'm pretty sure ur probably in the eastern hemisphere. Not positive, just pretty sure.