There are a lot of options when picking aquarium fish. I really do recommend you get the advice of a good retail store employee. Find somebody with experience. Somebody that knows the specifics about your aquarium. The size, what your budget is, whether or not you want fish that are going to outgrow the tank so eventually you'll get a larger aquarium.
Fish are very territorial, and it's been dubbed that next to people, fish are the most territorial animals on the planet. So keep that in mind when buying fish. It's very important to not buy two fish of the same family, because sometimes they're going to fight, especially with saltwater aquariums. With freshwater aquariums, it's a little bit more forgiving. The fish, I feel, aren't as aggressive, unless you get into the group of fish called cichlids. Those are very territorial, but they're also very rewarding and very easy to keep.
So to pick your fish, one of the main things is size. A lot of fish are not suitable for most aquariums simply because they just get too big. Now, there is that old fish tale that a fish will only grow to the size of its environment. Well, that is true to a certain extent, but it's also very cruel to put a fish that typically gets very large in nature and stick it in a very small aquarium. Yes, it won't get as big as it would in the wild, but its growth is going to get stunted. It's going to get very deformed-looking.
So pick fish that will match the size of your aquarium. And remember that your fish are going to grow. The basic rule of thumb is one inch of fish for every five gallons freshwater, and one inch of fish for every ten gallons saltwater. But factor in the growth rate. You probably will lose a couple of fish here and there. It is normal. So you always have a couple of fish waiting in your head that you want to add in the future. But resist the urge to overcrowd the aquarium.
And try to create a nice, balanced . . . we call it the garden effect. You want a nice, balanced garden. You want some reds, some yellows, some greens. You want to balance the colors. Or put a lot of thought into making your aquarium have a certain look to it. Maybe just red fish, just green fish. So you can have a lot of fun with the color aspect of the fish that you're going with.
But one of the main things is size. The two main things are size and aggression. But there are so many options when it comes to fish that for this, I really do recommend you don't buy the fish just on a whim or impulse or online, late at night. Get the advice of a retail store owner. Or call or email, if you're buying them online. A lot of the reputable companies online will be able to answer emails or even take a phone call and give you advice as though it's a normal fish store.