A friend of mine told me that she sucked at keeping fish. I asked her what she had and she said she had a bowl with two of "those betta fish things" and they killed each other. They guy at Walmart told her two would get along just fine! She thought it was something she did, apparently she had never heard them called Siamese fighting fish. Another instance of needing to do research before purchase, though.
A few more :
1. Just use aquarium salt to make brackish water. Even better, on the little "care" card for green spotted puffers at Walmart it says they prefer brackish water, so add 2 teaspoons of aquarium salt per gallon or something like that. Wrong salt, and not enough of it...
2. African cichlids should be fed feeders regularly because they are aggressive. Wrong, too much protein can kill them!
3. You should just change all the water every couple months instead of weekly partial water changes.
4. pH must be 7.0 to keep tropical fish, or to keep such and such fish you have to have a pH of such and such, and if your pH is any lower or higher you need to use chemicals to make it neutral/whatever exact number you were given. Fish are very adaptable and most, especially if they are tank bred, can and will do fine within a pretty large range of pH.
5. Products like ammonia lock or ammonia removing filter media are necessary for the health of fish. They really just stall or kill your cycle. Ammo lock can be pretty useful in a dire emergency, though.
6. Test strips are the best testing method because they are quick and easy and have less room for error.
7. Water changes are bad for fish...
8. You can put 5 fish in a little 2 gallon tank.