I have a fairly heavily stocked 125 running on a single fx6 and I feel like if it was more lightly stocked it would be great but right now not really enough, so for a 200g I wouldn't use it alone. as far as the filter itself I love it, super easy maintenance, the purge valve/hole at the bottom is awesome during w/c's and people have done diy auto water changer things with that feature. it is also self priming which is awesome, I came from using the other fluvals with the like plunger bit and this is waaaaaay easier. For the size of it pretty quiet. con is that when I close my hoses ive noticed a small leak at the bottom by the pump, but I haven't totally investigated that yet because its only some times and isn't a real big concern at this point. I'd give it a 8.5/10 filter stars