I like these kinds of threads. It gives aquarists a place to commiserate and new people can learn hard knocks without going through the trauma of losing a lot of fish.
I want to contribute these lessons:
1. Spend actual time watching your fish, like 30-60 mins. daily at least. If you really know your pet's normal behavioral patterns, you can easily catch problems like disease or systemic issues long before they become extreme or life-threatening. Learned from treating fish like "decorations" early on in the hobby and many preventable losses as a result.
2. Minimize sticking your hands on your fish tank. I only put my hands in there for feeding and for maintenance, and I'm sure to really wash them beforehand. I killed an entire 36g community tank from cross contaminating the aquarium with bacteria from a clogged sink drain by accident. Had to unlclog when siphoning water, absent mindedly didn't clean my hands before jamming them back jthe tank. Days later I lost 14 fish in a week. Really nasty stuff if not careful.