fish in cooler dying quickly w/ waterchanges

He is on the Potomac River, not the Elk, where Bluestone Dam is. Sadly, didymo (a very bad, invasive alga) was recently found below Bluestone Dam a few weeks ago.
As far as fish go. The water temp and air temp certainly aren't helping. I was below Dam 5 today and I belelieve we recoreded consistent temperatures of 28 C. We had quite a few species of minnows have high mortality today while in a large livewell system that has several airstones and gets regular (every 30 min) near complete water changes. The larger things, like redhorse, sunfish, bass, catfish were fine. It's hot, plain and simple, and fish are stressed. The cooler is farily small, the constant water changes are probably freaking them out, and the temperature spikes that are possibly probably aren't good either. As long as you can find some slackwater around aquatic plans or wood you should be able to catch all the spotfins and satinfin shiners you want in a few attempts. I say them everywhere the past two days. There were alot right off the boat ramp in Williamsport MD. There is a portage below Dam 5 off the canal, you should give that a try too. Alot of plants on the bank where the minnows are heavy.

thanks matt.
They are EVERYWHERE here, but here in the slackwater above the dam, the minnows and shiners are smart when it comes to my umbrella net and minnow trap. I catch more baby sunfish than minnows like 10 to 1.
they seem to swim around the net, and peck at the bread from the bottom of the net and trap.
any tips?
thank you for every thing matt, I appreciate it very much!

He is on the Potomac River, not the Elk, where Bluestone Dam is. Sadly, didymo (a very bad, invasive alga) was recently found below Bluestone Dam a few weeks ago.

Oh no I didn't hear about that, I'll have to look into that. We (wife, son, and I) were just below the dam yesterday.

I think you've gotten all the help there is...
...cycled substrate will help
...handful of carbon in a stocking or bag with you air stone
...cool air
...ammolock/stress coat/etc.
...some folks also add salt to the water to reduce stress (lots of threads on that)

...and that's really about all you can do once you have them.

Hope this isn't too late, I've been on vacation.

In addition to the excellent list above, in the summertime it is especially important to pay attention to what happens to the fish stream-side, between the net and the cooler.

* If you use "a collecting bottle" (say attached to your body) to keep them while you're collecting more, that's a small amount of water that will heat up quickly and have limited oxygen capacity. The fish will become stressed quickly, and perish later. More than 5 minutes in a 1/2 gal plastic bottle in the summer is probably pushing it. (Don't ask how I know this.)

* If you transfer the fish to a bucket, where are you putting that bucket? Sitting on the bank in the sun?

* Do a 90% water change with cool clean water on the cooler/bucket just before leaving the site. I like to think that I'm dumping most of the first round of stress hormones etc that have been released into the water, and using new water that they came from also minimizes the risk of any pH or hardness issues. (I also believe that for most fishes water temp and oxygen capacity are far more significant mortality factors than pH or hardness, but there are a variety of opinions on that.)

Also, (except possibly for very young fish) better to leave them unfed for a week until they get home than to attempt to feed them in the cooler, which just adds uneaten/eaten food byproducts to your water quality issues.

Did we mention aeration = good? Aeration = always good, whether by battery-powered bait-saver or regular air pump hooked up to a DC/AC inverter.

HTH