When you have completed the previous steps, you will have a tank filled with water, but you will not have the working, well-balanced artificial habitat for fish that we call an aquarium. This is just untested water in a tank. The filter hasn't had time to work through the cycling of the water. The heater hasn't been adjusted, which may take a few days. In short, while there's lots of cool stuff in your tank, it's not ready as an inhabitable place for your fish to live.

To achieve all of this, you need to let the tank mature. As you read in Chapter 1, fish require suitable water quality that has appropriate levels of water pH and hardness. Your tap water may be hazardous to your fish by harboring unknown additives (many municipal water supplies have treated water). And lest you forget, your brand-new tank does not have a well-established nitrogen cycle. Safe water parameters need to be established before fish can be added to the tank. Temperature regulation, filtration, and water circulation will help your water to mature. It just needs a little time.

Experts disagree a bit on how long it takes water to mature. Size and water condition are both factors that lead to those types of discrepancies. For example, a new under-gravel filter may take between four and six weeks to fully establish itself. You should see what the filter's manufacturer's directions advise.

However, if you don't have an under-gravel filter, most experts agree that you should probably wait a minimum of 10 to 14 days before introducing the first plants and fish into your aquarium. We have added fish and plants to a new tank in as little as two to three days. It depends on when your pH and hardness stabilize and on how well the nitrogen cycle is established. There are commercial treatments available that will accelerate the maturation process. Some of these actually come with your aquarium kits that are available at your local pet store. Check with your local dealer and read instructions carefully.

Before you can add fish, though, you will need to test the pH of the water. You'll probably need to do this once a day to gauge how your water is doing. You cannot add fish into a tank in which the pH is off. You'll only kill your fish or make life difficult for them. The pH and hardness should stabilize in two to four days. Then it is important to fuel the nitrogen cycle. This can be done by introducing a few very hardy inexpensive fish, like common goldfish, into the tank. You will need to produce the necessary ammonia that will help establish bacterial colonies that comprise the nitrogen cycle. To do this you need to add a few fish that can biologically and naturally add that ammonia.

Now that you have introduced fish, it's important to continue to monitor ammonia, nitrite, and nitrate levels. You want to be careful of “new tank syndrome.” At first, within a few days, you will see ammonia and nitrite levels begin to climb rapidly and nitrate will remain low. If you have introduced the fish too soon, this can result in the poisoning of your fish. This especially happens with many beginners who rush the maturation process. This means that the bacterial colonies which convert these compounds into the less harmful nitrate are not yet established. In most cases, if you wait a few days before introducing your “starter” fish, this will not occur. That's why patience is so important.

Generally, in about one to two weeks, your ammonia levels will stabilize. In this case, nitrite will decrease, and nitrate will increase. This is the sun breaking through the clouds. You can now add fish to your exciting new habitat.

This needs to be done slowly. You should add a couple of fish every few days in batches. Remember to calculate the number of fish you think your tank can handle, and buy conservatively. You need to think this through. As you continue to add fish, you need to continue to monitor ammonia, nitrite, and nitrate levels. If a sudden peak occurs in any of these categories, stop adding fish. When that peak diminishes, you can continue to add fish.