okay so uh I need some help-
I have this small planted tank stocked with planarians, copepods, snails, and amphipods. They've been thriving for several months, but I checked on them today and they're all dead??? There's a few survivors, but not many. The plants are fine though, which is kinda odd. My dad takes care of them when I'm at my mom's house, and I asked him if he'd put anything in the tank. He said he had topped the water off with conditioned tap water like he always does. I just tested the water and it says it's very alkaline and very hard. Does anyone know why this happened? Nothing new was added to the tank, I genuinely have no clue what caused this. And what should I do next? How can I prevent this from happening again in the future?