Wash Your Hands!
I bought some fish and snails recently at an out of town LFS. Instead of using a net, the woman plunged her whole arm in the tank to snatch some snails! That upset me because there were nets sitting right there!
So, did you wash your hands/arm before putting them into your tank? I’ve noticed a lot of instances where people are putting their hands in their tanks to feed or pet their fish! This could have drastic effects on your water chemistry and consequently the fish.
There could be remnants of substances/products on your hands and arms that you have used recently that are deadly to your fish? For example: perfume/fragrances; hand cream; shaving cream and regular soap/detergent, are a few. These substances cling under your nails or under your rings, then leach into the water.
If you need to put your hands in, you should wash them with plain soap (castile or unscented) not anti-bacterial, and then rinse thoroughly. Make sure you rinse them again before going to the next tank!
Just a pet peeve of mine!