Newbie Cooler (ice chest) live bait help?

[Seth420cr909]

Ok I have a 20-25 gallon cooler. I bought a aerator pump from bass pro that says it will convert any ice chest into a livewell. The pump is a 400gph that tubes up to the top and connects to a pvc pipe with slits cut into it so it shoots water down from the rim of the cooler.
Alright now on to my problem. I went and caught live bait (creek chubs)(Shiners)and(suckers) from my local river (Little Miami in Ohio). I took the water from the same river I caught the bait from.
I got approximately 15 shiners, 4 chubs 2-3in, 1 8-9in chub, I put all of the bait in the chest when I got home plugged it all in the first time I kept it inside my house I keep my house temp at 75 degrees, I was told to keep the water chilled so every 12hr I put a frozen 18oz water bottle by the bilge pump to keep the water chilled. I also broke up a half a piece of just starting to mold white bread in it. Also I kept the lid cracked to keep fresh oxygen in there over night.
The next morning I woke up there were 2 dead shiners and 1 almost dead shiner floating in the cooler, so I cleaned
ed the 2 dead out , there was still about 5 pieces of bread left in it so I went to work... Then when I got home there was 2 dead small chubs my big chub was also dead at the bottom and 4 dead shiners with bread still in there from the morning.(I work 6:30 to 3:15)
Ok after that I assumed that the cooler water might have got too cold.
That night I went out and caught a 6 in sucker and brought it back with a 7g bucket of river water, so I took out about 75%of. The cold water from the cooler and filled it back up with the water I got that day to warm it back to normal temp. This time I put. The cooler outside to try and keep the temp from getting to cold. I put a new quarter piece of bread in there that night.
When I got up the next morning the sucker I just caught was dead along with 4 more shiners.
I have no clue what I am doing wrong.
Is the pump to strong for that size cooler... I have no clue?
Any help would be much appreciated.