Snails.... seemed cool to add to my tank but the resilient buggers never die and 2 snails became an unreasonable number over night. Also my sister bought me an oranda when I kept gold fish a few years back. It ended up getting sick every two weeks. I rescued it multiple times in 5 months only to have it take the whole tank down, well ok it and 3 other goldfish. I didn't replace the fish and tore it down for storage. Met my wife at that time and was coaching elite level hockey, so I didn't have time to dedicate to new fish. Truth was I was ready to be out of goldfish and was considering giving them to a relative at the time. Wanted something else. Now a few years later here I am with a wonderful african setup with a wife who is really getting into the hobby.
Biggest rookie mistake (lots of those here). When I first got the goldfish. I got talked into it by an old girlfriend who had fish. She told me a 2.5G would house the little fancy we were looking at. So I asked the people at the store who had no problems with me carrying out a 2.5g tank, and a bag with a fish in it at the same time. Went home and started researching what I could do to help the little guy live a happy life and was out shopping for a more reasonable sized tank a couple of days later. Did a live cycle with the poor gold fish because I didn't even know that I needed the cycle. Somehow the little guy made it and he got a couple of tankmates who all grew fast in their new 40g. This is when I discovered MFK, an amazing site that has helped me explore this hobby. Wouldn't be the diligent fish keeper I am today without the people from this site and their expertise.