When I first decided to set up a marine tank after only having done freshwater I pretty much did things like I had before. It worked pretty well. As I learned more though I came to appreciate the value of a sump. However, I didn’t have a drilled tank with an overflow and couldn’t afford one. So I hit the forums and found someone local who had a used hang on the back (HOB) overflow for sale. I set it up and was hooked on the sump forever. But those overflows aren’t without their problems and mine was pretty darn large. This new product is a smaller version that also helps prevent the siphon from stopping which was an issue (as was noise) with the standard HOB overflow.
If you don’t want to drill, or for some reason can’t drill, then a product like this one is for you. They come in various kinds. I used an overflow box from eShopps for my first one.
The benefits of a sump are numerous. First off there is the simple aesthetic reasoning behind it. Having a sump means all your equipment is down there out of sight. Having filters, heaters, and other things in the main tank can be unsightly. Then there is the fact that it allows you to have other equipment, like a skimmer or a reactor of some kind (GFO, carbon, calcium). Also, the sump can be a refugium. That’s treating part of the sump (it can be divided for equipment in one area and a refugium in another) as a special area where you can put weak fish, extra live rock, or grow out algae like cheatomorpha or other kinds. These act as natural biological filtration for your tank.
Some people even make their own overflows. Eventually, I think most people gravitate to having a drilled tank with a built in overflow but you can have the same benefits without that by using a hang on overflow that you buy or if you are handy, that you make yourself.
The Popko Aquarium Siphon Device is an ingenious new take on how to get water to drain from your tank, without having to drill it. This miniature siphon is
Source: Hands On With The Popko Nano Siphon by Oceanexus News Reef Builders | The Reef and Marine Aquarium Blog