I've noticed also that my african cichlids have been breathing hard after water changes, and some have died recently. I also use prime right after new water is added.
Today, I tested the tap water, which is the primary source for new water, and found the ammonia is 1ppm. Before, it has alway been 0ppm. Don't know what happened, and always took it for granted that the tap water's ammonia has been 0ppm. At least two years ago.
So I suggest that you test your tap water, and check the parameters. May need to add extra prime to neutralize the ammonia in the water. I didn't bother to test for nitrite in my tap water since ammonia is already high in my opinion.
I'm also hoping some of the other members on the forum may have a solution to bad water parameters from the tap.