- All of our tanks are on the same system. While they might say, "Don't buy anything from that tank because those fish are sick," they probably won't mention that it really doesn't matter because all the tanks run off of the same filtration system - or that they just netted out a few fish with ick ten minutes before you walked in. This is one of the primary reasons you still need to quarantine new fish, no matter what.
- That goldfish can't live in a bowl. Goldfish get big - like larger than your hand big. None of those 10 gallon and under goldfish starter kits are suitable to house any of the varieties of goldies - even the 13 cent "feeders". A single fancy goldfish needs at least 30 gallons to live comfortably (although, a 30 still looks pretty crowded with one adult fancy, and you're still going to be doing really frequent water changes. Bigger is always better)
- I've never kept fish before. Many pet store employees at some of the larger chains don't actually have any hands-on experience, they've only read through a handbook. This is not the kind of fish tank advice that you want to follow, especially when it comes to serious matters, like medicating your fish.
- Don't add the bag water. You should never add the water from the bag that your fish came home it. Not only can you unknowingly spread disease and parasites, but you may also end up with a fish tank full of hundreds of baby snails.
- You have to wait longer than 24 hours to buy fish for a new tank. You can't buy a new tank, fill it with water, and then throw in a handful of fish if you expect them to live past their first day. Your tank needs at least a week to get a bacteria colony started - it takes 6-8 weeks for it to cycle. You shouldn't start adding fish until the first part of the cycle is done - wait until the ammonia stops spiking.
- Those fish aren't compatible. This is why it's important to do your own research before you buy a fish. Some fish will not get along, no matter what you do. Not all fish eat the same diet, need the same water temperature, or can even tolerate tank mates to begin with.
It's important to remember that a pet store employee is just a regular person, and often doesn't know much more about your fish than you already do - especially if they are a new hire. In other words, don't treat them like an expert until they prove themselves to be one. Don't walk in ready to buy a fish, or other any other product for that matter, without doing your research - you'll save yourself from a lot of costly, yet easily avoided, mistakes by doing so.