its probably a temperature thing, or a stress thing. I go through hundreds of minnows every month, and if water temps change between tanks, sometimes a lot of fish will perish. If they come in a plastic bag, set the bag in the tank and let it float for an hour (this will drive your turtle nuts), but that will let the temp of the water in the back acclimate to the water in the tank. Generally our turtle tanks are much warmer than the tank the 10 cent fish are kept in. Also this time of year, the drive home can lower the bag temp a bit. This wont guarantee all your fish will survive, but it will certainly help. Also, most RES will eat dead fish, but dont leave them in too long.
Also if you get your fish from a bait shop, beware of the blue water fish. If they keep their fish in blue water, they may contain chemicals that may be harmful for your turtle.
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