To get kinda technical, the theory of ,if you have a 20 gallon tank, and a 100 gallon per hour(GPH) filter, that your tank is turning over the water 5 times an hour is not actually correct.It would be almost 3 hours to make a complete filtration
Without getting too graphic with formulas and things, think of it this way.
Your tank takes unfiltered water, filters it, and mixes it back with unfiltered water.You now have a mix of the two, which eventually both go through again, and again, until 99.99 % is filtered. You will never get 100%, cause there will always be some amount that didn't get through in the period of time measured.
SO for your filter a Pen 170, without anything restricting its flow(filter floss,media, build up of sludge in the motor, intakes ect.) on a 20 gallon tank, it will take approx 2 hours for a complete turnover.
My tank has a 220gph and 350 gph filter(s), it would take at it's best about an hour and a half to 99.99% filter my tank.
Source=Aquatic systems engineering
I would get a pleco *celebratesmiley*