Here are pics of a bait tank I made a while back...it's actually two tanks; a 25gal sleeved inside a 30gal, insulated between and a Starboard top...the stand-pipe contains pump, air infusor and two-stage filter (filter not shown, but it's just batting and aquarium charcoal)...router-cut the top and both lids outta 1/2" Starboard...no thru-hull or over the transom plumbing...all it needs is 12VDC...
pump and air infusor in stand pipe
top stays put w/ 4 plastic wing-nuts
lid-stop...makes closing easier
Bait access lid
stand pipe/ filter access and air control
all closed up
Mostly used for freshwater Blue-Back Herring and also many times for saltwater bait; menhaden, mullet and whatever else comes back in the cast net :o ...