Yes, I agree with Rob. I have not yet run into a situation in which I needed to keep a fish longer than 15 minutes.... usually less than 10 minutes.
If I ever did though (with like say a record class fish) I would just pull the drain plug on the cooler and pour in a few buckets of fresh water every 5 minutes or so.

Speaking of coolers / livewells though, I think my 70 qt is a perfect size for just about any bass that I would be likely to catch..... heck, a WRB might only be a couple inches longer than my 18.4.... if not the same length, or even shorter, (she was 29" and the livewell size was fine) but on the other hand, I think I would like a 128 qt for giant Catfish and Carp. Hmmmmm..... "Captain, I think we are going to need a bigger boat" ! (smiles)

Peace,
Fish