Hey everybody, I have had a 20 gal and a 55 gal for a few years now and have had no problem with either one. I am currently looking into getting a much larger tank either a 125 gallon or hopefully something bigger. My question is what is an overflow exactly? I read something that made it sound like a built in filter...is that what it is? or is it something else just to circulate the water? Sorry if this is a dumb question I am just having trouble finding a straight answer. Thanks everyone!