Here you can ask questions (myth or not myth?) or post your own debunked saltwater myths!

I'll start it off:

You can't convert a saltwater tank to a freshwater tank or vice versa.

This is wrong. You can, and it's really easy. You basically just need to rinse and wipe the tank out. You have to be more thorough if you're converting from saltwater to freshwater though - obviously you don't want any residue salt in a freshwater tank!

Aquarium beginners shouldn't begin with saltwater.

I don't believe this is true. If a person's interests lies with saltwater fish than they should go with that. If you encourage someone to get freshwater fish when they aren't very interested in them than they probably won't be in the fishkeeping hobby for long. A fish-only tank isn't very complex and if it's a reasonable size (not tiny but not giant) than maintenance won't be too difficult.