I'll take a stab on this one, since I am building one right now.
An overflow is not a filter, it is just a way of getting water to your filter. This is usu. seen in
SW, but can be used for
FW as well. The basic idea is this: the overflow takes the water from the tank (by gravity, or siphon/gravity in the case of an external overflow) to a sump tank located under the main tank (usu. hidden inside a cabinet). A pump then returns the water from the sump back to the tank- this is the basic water circulation in this setup.
In the sump, you would put your filter (usu. a wet/dry) heater, protein skimmer, etc. so all the wires & equipment are out of sight.
The tank you are talking about prob. has an internal overflow. This is better than an external one but more expensive. An internal overflow has a hole (holes) drilled in the bottom so water can flow out. To prevent ALL the water from flowing out, there is a baffle/tube/overflowbox that regulates the water height. Only when the water gets higher than the set level will it spills over (overflow!) the box & down the bottom to your sump.
$300 is prob an average price for an internal overflow tank (often called "reef-ready" since this is the one to use for
SW setup). However, this is just the beginning - the overflow just gets the water out, you'll need a sump tank, a wet/dry or other filter, and a pump to get the water back up to the tank. Note the water has to get UP a few feet, so you need a more powerful pump - the usual powerhead won't do - more $$ ....
To get a pre-made sump/wet/dry at Big-Al's in town is around $200, pump another $100-200. Then you need plumbling parts to connect everything together, $50 or so. You'll prob. have to add another $50-100 for media & other misc. stuff. Gets expensive real quick ... that's why I am building my own ...
One bit of good news ... all my prices are in Canadian dollars - which is only worth 72 cents!
