Hello again Rob,
What I've got on my boat is a built in tank - about 20 gallons. This is filled quite simply by dropping, as you suggest, a bilge pump over the side and plugging in to one of the cigarette lighter sockets in a ring around the boat.
While fishing at anchor or on the drift, no problem, it just pumps fresh water into the tank which overflows out through the hull. The same pump is used to empty the tank, and with the aid of a garden hose connector can be used for hoseing down, washing fish or whatever - very versatile!
BUT, if you're rolling along at about 25 knots, you can't have a pump hanging over the side, and sand eels being sand eels, they seem to want to suffocate in very short time if the water is not changed regularly. Also, I know the local marks for sand eel trawling, but wouldn't know where to start if we were travelling up into Scotland or down into Wales for a fishing trip. With a good aerator pump, working off the same sockets as the bilge pump, maybe I could go out locally the day before our excursion, load up with sand eels and keep them fresh and happy in the tank overnight, while towing along the motorways, or whatever. That's the theory anyway, but I can't test it until I find a suitable 12 volt aerator.
I've looked at the small compressors from the likes of Halfords but they're either too bulky or too noisy.
Somewhere out there, somebody must be able to put me onto a simple aquarium type aerator - but 12 volt powered!
I've just had a thought - fish farms, etc, must use something similar, but perhaps on a larger scale, for transporting fish by road. Perhaps there is light, or should I say bubbles, at the end of the tunnel!!!