It sounds like a number of factors; overstocked, too much cleaning of the pond, possibly not enough water circulation or filtration, not treating city water before refilling.
When you clean a pond too thoroughly, you can remove some of the bio-film that is responsible for the nitrification cycle that processes the waste that your fish produce.
When you don't treat your city water before you expose the fish to it, chlorine and/or chloramine will damage the fish's gills which will affect their breathing.
If you are finding dead fish mainly in the morning, it could be because they died from a lack of oxygen during the night, this could be from overcrowding plus any damage to their gills.
When you can provide the information that others have asked for, we can come up with what would be best to do from here.
