On our (Wife & I) first 20gal setup, we ended up with 3 guppies, 3 platys, 2 clown loaches, a red tail shark, a glass catfish, a striped raphael catfish, and 2 mystery snails.
I didn't know anything about water changes, dechlorinator, cycling or any of that. I knew we had to let the tank run fishless for a week or so before adding fish, and the LFS sold me some nitrobacter (I guess its like stability).
So eventually the clown loaches start showing signs of ammonia poisoning and I think its some infectious disease. So I decide we need to remove the diseased fish and then the water changed out and the decor & gravel cleaned off so that the other fish don't get it.
We fill two mixing bowls with water to keep the rest of the fish in while we go about this process. Empty the tank, clean everything in tap water, refill the tank with tapwater at some random temperature and start adding the fish back. And by, 'some random temperature' I mean that I'm pretty sure we ended up using everything from freezing cold to scalding hot by the time we were done.
About 3 minutes after we put the fish back in the tank we notice they're losing all color and sort of floating around struggling to live.
They didn't make it through the next 15 minutes
After that I took the fish-keeping reigns from the wife, did my research on what it actually takes to keep fish and we've been happily chugging along since then.