I changed my filter from a Tetra Whisper I've had running for years to a Fluval 405 (used but cleaned out). Everything was working perfect yesterday, water was crystal clear.
Over the night, I heard crazy splashing. I woke up and found 6 of my fish dead. They are all peaceful community fish that I've had for almost a year now. The water was also cloudy, unlike yesterday where it was clear.
I'm about to test the water right now.
WHAT COULD HAVE HAPPENED?