Difficult Fish In Your Fish Tank
I wanted to talk a little bit about fish that might look tempting in the store, but should probably be avoided by beginning aquarists. All of these fish have their good points too, or the fish stores wouldn't be stocking them at all, but it's best to have a little more experience before adding these, or setting up a new tank for them.
If I start with goldfish, you're going to say "What? Goldfish are difficult? Since when?" Well, they certainly can be if you try to put them in a tropical tank with other fish. First of all, goldfish like colder water than other tropical freshwater fish, and trying to compromise on a temperature will make both sets of fish unhappy. Secondly, goldfish are DIRTY – they add too much ammonia and other wastes to the water for a delicate biological filtration setup to handle, and will require you to do a LOT more tank scrutiny and maintenance if you don't want a catastrophe on your hands. In short, get them their own tank and you're good to go.
Mollies are sweethearts, and very social, but they tend to do well in water that's a little more brackish (that is, salty) than what other tropical fish like. Like the goldfish, they'll be happier in a tank that's set up specifically for their needs. Setting up a brackish water tank is a really interesting process for the dedicated hobbyist, but again, not the best first stop for the beginner.
A discus is a beautiful fish, but they can be very expensive, which means that any newbie's errors you make in water care, maintenance, or overcrowding are going to be very expensive for you too. I'd recommend the inexpensive angelfish instead, which are similar in shape and a little hardier.
Birchirs are fun, but you need to remember that they will eat any smaller fish they can catch up with. Unfortunately, a birchir can grow quite rapidly, and fish that weren't "smaller" when you introduced him may soon find themselves in danger. These guys are also famous for jumping out of the tank.
The plecostomus is an amiable and hardy bottom feeder, but they also grow quickly, and most will outgrow the standard 40- or 55-gallon tank long before you're ready to start a larger one. Similarly, Bala Sharks and Hatchet Fish can grow to up to 10-12 inches long before you know it. With any fish it's important to learn the maximum size they'll grow to, and the speed of growth, before you lay down your cash.
Like with everything else about aquariums, the key to choosing the right fish is to do all the research you can before adding anything new.