Have you tested the tap water for ammonia, nitrite, nitrate & pH?
If not, then test it.
If you have smelly tap water then contact your water supply company and inform them. It could be something that harms people and animals if they drink it or get it on their skin.
You can also filter the water before using it in the tank. One of the members on here (Cowgirluntamed) was losing fish and uses well water as their water supply. They are filtering the water in buckets before using it for the fish and their fish have stopped dying since they started filtering it. They use carbon and zeolite in the buckets of water for a couple of days and then use the filtered water for the fish.