The thermocline is an layer in the water column where the temperature rapidly changes. This occurs during the warmer months when the surface of the water heats the the area below does not. These drastically different temperatures cause the water to no longer mix with the result that the colder, deeper, water can become oxygen depleted. While carp are highly tolerant of low oxygen, they are not happy in these conditions, and what is more their food is not happy and tends to smother. The result is that while the temperature might seem ideal everything else is not – no food means no carp.
The area right above this is where there is oxygen and the water temps are the lowest. This is where the carp will often be the most comfortable and importantly have plenty to eat. Where this zone is is highly variable from lake to lake. (Rivers are not affected as the moving water keeps things mixed.) In general the clearer the water the deeper the fishable zone as photosynthetic organisms (plants and algae) can produce oxygen deeper in clearer water.
Finding how deep you should fish is the kicker. If you pull a lead along the bottom and find some vegetation you are not too deep as these plants will produce oxygen. In many lakes a good way to find out how deep you can fish is by looking at the fishing report. No, you will not see a section titled “carp” but you may see how deep the bass or the trout are being caught at. If trout can survive carp will be doing great and you can feel confident in fishing at that depth. You can also talk to the local fisheries biologist who should be able to give you idea of the “temp/DO” profile of the lake. After that get out the marker float and find where this depth is.
So what is you see fish crashing in open water? Maybe try a zig rig. If you see carp crashing in water that is deeper than the thermocline, and you know roughly where the thermocline is (from fishing report, etc) why not try a zig set to just above it? I have seen first hand while diving massive amounts of zooplankton that can be found in this part of the water column in really deep water and if I have seen them the carp know they are there and will focus in on them. Knowing this might make the difference between blanking and banking.