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!

Used in saltwater setups. I can draw a picture, just can't explain it very well.

If the type of overflow you are talking about is one you can add to the back of your tank it is basically siphon driven with 2 chambers, one sits inside the tank and sucks in surface water through a "comb" like grid with a prefilter sitting against the front side-the water flows in and is taken through the siphon tube to the chamber on the outside of the tank and then down into the sump where the water is filtered and then returned to the tank by a submersible or external pump.

I have also seen overflow attachments for some power and canister filters. These are usually round in shape with the same "comb" grid but hook into the intake line of the filter and allow the surface water to be skimmed rather than the filter only drawing water from the bottom of the tank. They usally come with an adjustment bar to control the amount of surface skimming so the filter can draw water from both the bottom and the surface.

You also have aquariums with built in overflows which are usually drilled tanks and most often also require the use of a sump setup.

I hope this description helped you and wasn't too hard to follow.