#1- The more fish, the better!
This is absolutely false. Overcrowding your tank, one of the mistakes made most often by first-time aquarists, is a recipe for disaster. One of the most common stocking errors is to fill a 5-gallon tank with upwards of 30 fish—yikes! Like people, fish need room to move. There also needs to be enough oxygen available for all of the fish you put into your tank. When stocking your tank, think "less is more."
#2- The smaller the tank, the easier it is to keep.
Many people believe that a small tank will be less trouble: it will contain less fish, take less time to clean, and generally be less of a commitment. Well, the truth is, a smaller tank will be just as hard—if not harder!—to maintain as a large tank. In fact, many large tanks are actually easier to care for because, if a problem arises, you'll have a bigger window of time to try to solve it before disaster becomes inevitable. A smaller tank, on the other hand, can quickly be overtaken by algae, disease, or a variety of other things. Also, it is much easier to maintain appropriate levels of pH, alkalinity, etc, in a larger tank because fluctuations will be minimal and easier to stabilize.
#3 Taking care of a tank of fish is a walk in the park compared to caring for a dog or cat.
Sure, your fish won’t wake you up in the middle of the night to be let outside. And your fish won’t scratch claw marks into all your favorite furniture. But fish certainly have their own set of needs, and taking care of them can actually be more challenging than caring for a dog or cat simply because they are unable to let you know when they need something. If you think you want a fish tank, make sure you’re ready for the commitment and not just looking for an "easy" pet.