Here's the basic concept of a recirculating tank at work:
All the tanks are plumbed to one of several sumps. Unfortunately it is virtually impossible to get pictures. I tried this morning but there are tanks and tubes in the way. That's why I posted an old photo of the rack. Where I took that picture several years ago is not a 300 gallon Cube tank.
The basic concept is a tank is plumbed via bulkhead to a drain that leads to a sump. The sump is where your filtration is which leads to a pump and UV. The water is pumped from the sump to the tank, overflows the bulkhead and returns to the sump.
Here is a goldfish tank I installed for someone, I built the filter out of a 29 gallon tank:
http://picasaweb.google.com/DavidLains/JoeSGoldfishTank#
This is my main filtration sump which is plumbed to drain so it can over flow. Water can be added to any tank and the sump over flows to drain:
All my larger tanks (and sumps) are plumbed to a system out one bulkhead or to drain with the other. This allows clean or dirty fish to be isolated or the fish to be kept on system or in a large water change style.
Here’s the main sump when it was being installed:
A bunch of pictures of the last major addition a year or so ago:
And a few various pictures of a trout tanks system and etc:
