Gus, your explanation shows how I did not understand this correctly.
And I do not want to take much time and attention away from this topic (i.e. not hijack it). But let me explain where I was coming from a little more.
For my tank, which is fresh water, on the boat I have a pricey, recirculating 35 gal bait tank. So far I have done pretty good keeping bait alive, even during the 90 degrees of summer. I use many ice jugs to keep the temperature on the boat tank below 68-70 degrees. I cannot afford for it to get too much lower, because I have to be aware of the lake water temps I will be putting the alewives into. Just like when you bring the tropical fish home from the pet store. You should let the water temp stabilize. I just go thru a lot of bottles of ice which I carry in a separate cooler. I also use them for my catch cooler should I be so lucky as to actually catch a Striped Bass or two. The salt idea for me was that maybe I could carry fewer jugs. But that really is no big deal.
Now when I get back to the dock, I transfer the remaining bait back to my large 100 gal holding tank. Now this one is the one that gives me a challenge. I have a recirculating and filtering system that works great during the cooler days. In cooler days, I can keep the temperature below 70 degrees with a minimal amount of work. But in July and August, times are tougher. First thing I try to do is to pump lake water pulled up from 35 feet at about 65 degrees and refresh the holding tank. I do this about every 24 hours and it brings in fresh and cooler water. The rest of the time, the tank water is recirculated and filtered. But the sun does bear down on it. I am going to add an awning which will help some.
Here are two pictures of my tank:
But one great idea I got from this forum is to take a 5000 BTU unit which I am getting at a low price used and do like the TD campers do. I will have it outside of the box with incoming and exhaust tubes to cool the ambient temperature in the box. I might even experiment with a small tube to divert some cooled air directly into the bait tank. My unscientific logic is that add cooler air into the water cannot hurt and should have some positive effect on the overall water temperature. Anyway, I like to experiment with things like this. And the AC unit itself will finally go to my trailer once I am ready for it.
Bob