A few I can remember. When I was still onto goldfish, I do not know about cycling. I got an ammonia spike and one by one my fish are dying and I have no clue why or how. When I went into cichlids, I still did a lot of mistakes. On my 125 gallon, I bought a green Texas cichlid and then 2 peacock bass though a month apart. The stupid Texas made hell out of the peacock basses' lives. They cannot move an inch without being terrorized by the green Texas, bites them in their eyes. I ended up giving the bass back to my LFS. Then I got a 250 gallon and got a peacock bass that is about 3x as large as the Texas. Thought that would reduce the aggression, no! The little Texas does the same, bites the larger guy in the eyes. This is where I became an idiot. I used a few feeder goldfish to try if the glue on the custom tank has dried. If they die then it has not. 7 days and as many as 10 feeders later, the tank is still smelly. Back in my 125 gallon, the Texas is still a bully. Got tired of it, dipped him in the 250 gallon. Every time he is close to dying, I put him back to the 125 gallon. After a few thousand gallons of water and this trial and error where the Texas is dizzy but eventually okay, the 250 gallon is deemed ready for occupation. The stress caught onto the larger peacock bass and his fins started rotting. Idiot me put him onto a tank full of strong medicine but he didn't make it. The irony, I lost 3 peacock bass for a very common green Texas. Later, I bought a red devil and a giant gourami. This proves to be another mistake followed by a week of sleepless nights where my other fish (Oscars, even a 2' silver arowana) are being chased and nipped. I almost gave up on the hobby. In the end, I exchanged the giant gourami and the red devil each for a peacock bass. I even added the green Texas cichlid for free just to get rid of him since his attitude has not changed a bit. The LFS added him onto a tank of >20 red devils. A week later when I went for my supplies, the Texas is in a corner, almost dead due to aggression. Forgive me but I hate that fish with all my guts. Sorry for the novel/long post...