After you've assembled the perfect aquarium, you're ready for the fun part - adding fish. Here's what you need to know about populating your new tank. Try to establish a community environment with several species who will coexist peacefully. The rule of thumb is one inch of freshwater fish and half an inch of goldfish and saltwater fish per gallon of water. This rule is based on adult fish size. If you try to squeeze in a few more roommates, you're just asking for trouble. Too many fish leads to stress, pollution and disease outbreaks.

Getting Started

When stocking a new tank, add only a few hardy fish who are able to withstand moderate elevations of ammonia and nitrite until your tank builds a healthy nitrifying bacteria population. These naturally occurring bacteria help control waste. Tetras, zebra danios, barbs and common goldfish can best handle the ups and downs of a fledgling filtration system. During the first month, perform a 25 percent water change each week to help dilute contaminants and keep the tank fresh. If you prefer larger fish, such as oscars, keep only one or two in a 30 to 50 gallon aquarium because they are territorial and might bully their tank mates. But they make up for their independent ways by interacting more with their keepers. They actually act excited to see you and beg for treats!

After this initial 30-day break-in period, most aquariums are ready to welcome more fish. But don't push your tank's limits.

Choose Compatible Species

Your fragile guppy may not mix well with a fierce cichlid so before you drop them into a tank together, consider each pet's general behavior, feeding habits and territorialism. Fish are usually classed as community, semi-aggressive or aggressive based upon their compatibility.

More aggressive species, like tiger barbs and cichlids, are apt to nip fins or devour their smaller tank mates. And some fish may prefer certain water conditions. For example, livebearers prefer slightly brackish, hard water conditions while tetras like soft, slightly acidic water. Study the species you want, and only group those with similar habitat and water preferences.

Find Healthy Fish

After you've decided which species you'll cultivate, you'll need to choose healthy specimens. This is not always easy. In fact, most pet store owners can't predict which fish might develop disease. So all you can do is choose the healthiest-looking fish available.

Avoid selecting fish from an aquarium that contains even one unhealthy-looking tank mate. Watch the fish swim. Are they active, alert and able to maintain an upright position? Some catfish varieties swim upside down, but they should do so vigorously.

Examine the skin, scales and fins for lumps, bumps, open sores, white or yellow spots, redness and ragged tails or fins. Fish who hold their fins close to their bodies, breathe rapidly and seem reluctant to move from the water's surface or aerating bubbles may be suffering from an infectious respiratory disease. Healthy fish should look plump, but the scales shouldn't flare out. Abdominal enlargement and a gaunt hollow belly signal potential disease. Stick around to watch the fish eat. Do they quickly dart to seize the food? Sick fish often lose their appetites and exhibit less gusto at mealtime.

If you're planning to invest in a marine specimen, visit the store several days in a row to ensure the fish maintains a healthy appearance and appetite. Don't rush the purchase - no matter how rare the fish is, you don't want to jeopardize your other pets' health with an unhealthy addition.

Another precaution should be to always quarantine new purchases in a separate tank for at least three weeks before adding them to your aquarium. Most diseases will appear during this time, so you can treat infected fish without harming their future companions. To keep potential pets healthy, high-volume suppliers often take routine precautions such as deworming fish and administering saltwater baths and antibiotic treatments.

No matter how eager you are to put the finishing touches on your aquarium, select your finny friends carefully - their lives depend on it.