Here's an installation that one of our members (Adam F) did on his Warrior 165.
To start here is the barrel. It came from here: http://www.smithsofthedean.co.uk/Miscellaneous.htm
(60 ltr - a 120 is an over kill, the 60 will hold 200 eels or about 30 macky).
I have both the inlet and the outlet at the same level. Many choose to have the inlet lower to create better water movement, but for some reason I find this works better. Inlet is 3/4" outlet is 1 1/2" needs to be bigger to keep up with the inlet. The tap at the bottom came from a self brew shop in Ringwood for £1.50. This allows me to drain the tank without having to heave it over the side. The water is drained into the boats bilge and pumped out again. I have a webbing strap that secures the tank to stop it tipping over. Finally these tanks come with a lid to stop bait jumping out, to stop the water sloshing when under way and to sit on when not in use.
The pump is a cheap Rule 350 GPH bilge pump coasting about £10. This way it is cheap enough to replace when / if it breaks. So far it has seen a years use on the back of the boats and still works and looks like new. I hunted down a side mounting bracket from Christchurch Boat Shop for about £3, and siliconed and screwed it permanently to the transom just under the water line. It sits here all the time and doubles up as the pump for the deckwash. The tube runs in through the well via a 'witches hat' and the cables are routed to a waterproof switch on the dash to make it easy to switch on and off.
...and finally: I installed a new skin fitting alongside the current outlet for the main bilge. It is here that the water exits the tank.
The two hoses fit inside one another so when the tank is not onboard they store by sliding them together underneath the transom - keeps them out of the way. The deck wash hose fits snuggly inside the inlet hose for washing down although I am trying to find a plastic Y connector with a diverter to allow me to run both the tank and the hose and switch between them.