When I fished out of CT I use to keep Eels in a live well for 2-3 weeks at a time. I know some people like to ice them but I would keep them in a well that had a pump on a timer. I have 4 batteries so I would turn two of them off. If I was off the boat for more than 2-3 days I would throw them in a pen over the side. I found they would turn grey if I did not cover the tank but this would take a week or so. While down there we fished Eels 95% of the time and never wanted an excuse not to wet a line before or after work. I even had other friends at the dock that would throw some eels in if they were going home for the week and had some left over. I've tried this up on the South Shore with Mack's, Herring and Pollock but had to keep the timer on such a short interval that I felt better throwing them in the pen if I wanted them to stay over 24 hrs. I think the less light the better but have a solid lid so I don't know if clear would hamper them.
I found that the Mack's need lots of flow to keep the water clear of blood. When there running and were catching them I only throw 10-15 in the tank at a time. If I catch more I throw them in a bucket and freez them for chunking, Of course I always throw a hook in them and give them a swimming lesson to help keep the numbers down... I added a pickup this winter to help this so when the boat is moving I don't need to run the pump. We'll find out how this works in about 99 days! (And Counting!) Water temps off of Cape May still only 39 Deg...
Capy May Bouy