funny. when i look at the specs of a 170 it say 30 gallon max.... I doubt that it could handle a 50.
Why should you get another filter.... ok. pull out your filter cart ill wait....
Ok. now see how little media is actualy part of the "filter". just a little bit of thin sponge over some carbon. you can put the cart back now... but clean it out real good, that might be a problem too.
Other filters have more media for filtering. ie the aquaclears have large sponges that water filters through. Or if you had a canister filter like say a Eheim or a Filstar xp then you would have like a foot of media that water needs to filter through before it leaves the filter.
What is the total output on that 170 (gallons per hour) if its not at least 10x your tank volume then its not enough.... and everyone here will tell you even if it is 300 gallons + that more filtration never hurt anything.
hope that helps explain why another filter would be a good idea.
Lev