What is the worst disaster that ever happened in your tank? In my case, it was a population imbalance between males and females that led to the destruction of my cherry shrimp population.
I’d have to say when I adding in a pretty badly attacked black knife eel as a friend was tearing down their tank, only to notice a day or two later that it completely killed all of my other community fish in the tank (not sure if it was an attack feast, or if it was super hungry, or what). Kinda sucked the most as doing a favor tended to make me restart my whole tank.
Probably the worst thing for me was a heater malfunctioning and basically shocking or overheating my fish while I was sleeping.
Definitely not a fun time getting them all out while also getting shocked slightly until I found the piece of equipment that caused it.
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So many early mistakes come to mind, not really sure. A lot of my fish died when I was a beginner.