Need advice, carp dying

[Skippy]

Hi everyone. I know this is a bass fishing forum but I am hoping for some advice urgently please. 

Friends of ours own a farm with 2 dams and said we could come carp fishing in the one on condition we transfer the fish we catch to the smaller dam because he would like some more fish in there. 

The first day we fished we transferred one fish, the same the second day and om the third day we transferred 3. On the 3rd night though, about an hour after adding 2 of the 3 fish and coming back to transfer the final one we noticed fish floating and thought they had died either from water that was toxic or the change of environment (water temp etc that the transition was too fast). So we told the farmer we will not be transferring more fish because 4 of the 5 had died. They agreed and said we were still welcome to catch and release then into their larger dam.

This evening we went fishing again for the first time since then and caught 2 fish (about 20min apart), bringing them in quickly, removing the hook with a pair of pliers and in 15-20 seconds having them back in the water and off they swam, no problem. We caught the last one about half an hour before going home. As we packed up to leave, we noticed a fish floating in the water, still alive but barely. Went to see if we could help it but it died. A couple of minutes later the second one started floating. I am so confused and know it is not something we did wrong because we went fishing this past weekend, caught 20+ fish with the same baits and techniques and handled the fish in the same way, getting them back into the water quickly and those were all fine, we fished for the whole morning, no problem. So I am at a loss. I love the sport, but it really puts a damper on things when the fish you catch keep dying. I am totally clueless though, what could it be killing the fish? Is it possible that they are just dying from shock? If so, why would these be so sensitive compared to other dams we fish at? Please, does anyone have any suggestions?