Well, in the interest of "learning a lesson", I thought I'd post what I think happened to make my tank a Death Zone.

#1: Don't Change the Water

Okay, I admit it, I was a slob. And it's a bit easier to get lazy if you have a larger tank. But I really shouldn't have gotten so complacent with goldfish in the mix.

#2: Overfeed

Because I had so many varieties of fish, many with different food needs, I think I put too much food in the tank. Cloudy water = bad!

#3 Forget to plug in your heater

This wasn't a killer per se, but when the fish were already weak, I think this did off with a couple.

#4: Get mad at the aggressive fish

Yeah, sometimes you get a punk fish, but the mollie was aggressive because the other fish were getting sick. It wasn't really his fault; it's instinctual. Now that everyone's better, he's pretty mellow.

Anyway, thanks again for all your help, especially Apiladey!

I'm thinking very carefully of what new fish I'll get. I already got a new betta who is settling in nicely (I can't resist bettas. Pretty, cheap, low-maintenance...) Some sort of algae-eating critter, a catfish, maybe one or two more mollies. Maybe a platy or two?