Well, there are many measurements they go by. Some name their filters by the tank size. IE, a filter named "40" is good enough for a 40 gallon tank. Others, like "170", is named after the gph
that filter is rated at. Either way, many of us go by this rule. You should try for at least 6 times the size of the tank. So for a 30 gallon, you want to try and filter at least 180 gph
. If your filter does actually put out 170, then I wouldn't worry about getting a second filter to supplement right away. But see how it goes.