Book Excerpt: Your First Aquarium

Featured Article

Issue: November 2013

Author: Jay Hemdal

Photographer: Nikita Turnov/Shutterstock

In this TFH-exclusive preview of the upcoming Animal Planet book Your First Aquarium, a professional aquarist guides beginner hobbyists through the process of choosing fish and adding them to a new tank.

Planning Your Collection

The very first fish you buy for your aquarium sets the overall tone for your tank and determines what other types of fish you will be able to add later on. Some fish are peaceful and many other species will get along with them, while a few fish are so aggressive that they can’t be kept with any other fish. Some fish have specific water quality requirements that other fish do not share—you would have difficulty keeping brackish water mollies with soft water tetras, for example. Do you want to see shoals of small fish drifting peacefully through aquatic plants? If so, you need to remember that big fish often eat little fish, so you won’t be able to keep an African cichlid in that same tank.

You should visit your local pet store to see what type of fishes they have in stock (although this will change from week to week). Make a list of the species you are interested in and select one or two types to start with.


How Many Fish of What Size?

A very common question asked by beginning home aquarists is how many fish a tank can hold. This is called the stocking rate. This rate takes into account the type of fish being considered, the swimming needs of the fish, as well as the number of fish and their potential adult size. Aquariums can be overcrowded in three ways. There is a biological limit where there simply are not enough beneficial bacteria present to detoxify the amount of fish waste being produced. If the biological limit is exceeded, the ammonia and nitrite levels will rise, poisoning the fish. Aquariums also have a territorial limit where the addition of certain other fish will cause serious fighting among the inhabitants. Finally, there is a swimming room limit. An aquarium must have enough room for the fish to swim around and exhibit normal behaviors.

Biological Limit

There have been many attempts to answer the basic question of how many fish a tank can hold, but most of them are flawed. You see, they usually give this stocking rate as “so many inches of fish per gallon of water.” This actually compares two unrelated units of measure, length and volume, so it can’t work. It would be like saying that a car gets 100 pounds to the mile. As a fish’s length increases linearly (from 1 to 2 inches, for example) the volume goes up by the cube, changing the ratio. The example that makes this clearer is that while you might be able to keep twenty 1-inch tetras in a 24-inch-long, 15-gallon aquarium, just try putting one 20-inch pacu in the same tank—all of the water would splash out! Even though the “inches of fish” were the same at 20 inches total, the single 20-inch fish weighs about 70 times more than the 20 smaller fish combined!

Another factor that needs to be considered is the effectiveness of the filtration system used. An aquarium with an efficient filter will have a higher biological limit than a tank with a small economy filter. Finally, the water exchange rate affects the aquarium’s ability to properly house fish. A tank that gets a 25 percent water change each and every week is going to be able to house more fish than an aquarium that only gets a water change once every month or two.

So what method can you use to determine the number of fish that an aquarium can house? Fisheries biologists often use the weight of the fish per gallon of water (since fish weigh as much as the water they displace, it is the same as comparing volume to volume). For a normal home aquarium, a normal value for this is around 1 gram of fish weight per gallon of water (0.27 grams per liter). Crowded conditions would be seen at a rate of 1.5 to 2 grams per gallon (up to 0.54 grams per liter). Obviously, you cannot easily weigh your home aquarium fish. The best that can be done is to see examples in the sidebar that have been shown to work, and then try to duplicate them. Remember that unless you buy them as adults, your fish will grow and require more room as they get older.

Swimming Room for Fish

Swimming room is also an important factor for most fish—they need to have room to swim, turn, and interact with each other. Many of the calculations that have been used have the same flaw as mentioned before: stating that a 4-inch fish requires 30 gallons of swimming space, for example. Again, this is comparing a fish’s length to water volume, and it cannot work. What works better is to measure the open water length and width of your aquarium and add them together. This gives a linear measure. The height of an aquarium is usually proportional to its length, so you don’t need to be concerned with that measurement. Next, determine the adult captive size that the fish is expected to grow to.

You can either get this data from aquarium books or estimate it by looking up the fish on The final step is to make a ratio of the fish’s length to open water swimming room. If the fish has a maximum adult size of 4 inches and your tank measures 30 inches long and 12 inches wide, the ratio would be 4:42. You need to reduce this so the first number is a one. To do this, divide the second number by the first number (42 divided by 4 = 10.5). Your working ratio is then 1:10.5. These ratios work the same whether you are using English measurements, as in this example, or metric units. Just be consistent with which you use.

Armed with that information, you then determine which of the three categories your fish belongs to in the chart above. If you are in doubt, select category two as a compromise. The final step is to compare the ratio you calculated to the minimum ratios given. Make sure that none of the fish you want to buy will fall below the minimum ratio. You do not need to calculate this ratio for every single fish in your aquarium, just the one that has the potential to grow the longest.


Determining which fish will coexist peacefully in an aquarium is more of an art than a science. The basic compatibility chart (see p. 54) is a useful starting point in selecting compatible fish. Understand though, that in the end, fish are individuals and not always going to follow any set of rules. Even fish that have lived together peacefully for years can suddenly begin fighting. It is always best to have a backup plan for what you can do should your fish stop getting along. If you have multiple aquariums, you may have enough options to move your fish around as needed. If you have just one tank, you may want to invest in a tank divider. These plastic perforated plates are used to keep incompatible fish separated in an aquarium while still allowing water to flow from one end of the tank to the other.


Selecting Individual Fish

Now comes the time for you to visit your local pet store and buy your first fish. You may want to confirm your fish stocking plan with store employees and gain their approval. While this book gives you good general information, the pet store employees can offer you specifics about the fish they have in stock.

In choosing an individual fish, you should look at all of the fish in the tank to ensure they are healthy. One sick fish can very easily infect all of the others because fish diseases transmit so easily through the water. Look especially closely for signs that the fish might have ich. The fish should be actively swimming, without ripped fins or other wounds, and their eyes should be clear. The tank should be clean, and the water should be clear, not cloudy.

When it comes to selecting what individual fish you want to buy, remember that the pet store employee may have difficulty in capturing a specific fish from a tank full of identical species. The general rule is that if there are big differences in the fish (differently colored fancy guppies, for example), or if there are less than six fish in the tank, you should feel comfortable pointing out exactly which fish you want. For tanks filled with similar fish, it is usually best to first observe them to make sure there are no damaged fish in the tank, and then let the employee pick the fish out. Remember, though, that the fish most easily caught may also have some problem that made it slower than its tankmates! Watch how the employee catches the fish—did they have to chase the fish all around the tank? Did it take them a long time to make the capture? Did the net come up with lots of gravel in it as well as the fish? Did the fish flop out of the net and onto the floor? All of these things show that the capture did not go well, and the fish may have been severely stressed. In such cases, you may want to ask for a different specimen.



Whenever a fish is moved from one aquarium to another, it undergoes some degree of stress. If the difference in temperature or pH between the water in the two tanks is great enough, the fish could die from shock. Acclimation is the process of allowing a fish to gradually adjust from one water type to another.

Your new fish will typically be placed in small plastic bags at the pet store. Usually, a third of the bag is filled with water and the rest with either air or oxygen. Then the top is tied in a knot or sealed with a rubber band. If the store uses air to fill the bag, you should plan on getting your fish home within 30 minutes or so. If the store uses pure oxygen, the fish can remain in their bags much longer, even over 24 hours. During transport to your home, the fish need to be kept in the dark and at the proper temperature. Once you arrive home, the physical acclimation process can begin. If your pet store supplies you with instructions on how they want you to acclimate their fish, you may want to follow them. Otherwise, proceed as follows:

1.     Turn off the aquarium lights and float the bag at the surface of the tank. After about 15 minutes, the water temperature in the bag will be the same as the water in the tank.

2.     Open the bag and carefully pour about half of the water into a container. Take care that the fish doesn’t slide out during this process!

3.     Next, turn the top of the plastic bag down and in on itself a few times to form a floatation collar. You may find that the bag is less likely to tip over if you use a clip like a clothespin to attach it to the side of the tank.

4.     Add some water from the tank to the bag. This amount should be approximately 25 percent of the volume of water that was in the bag at step 3. Wait five minutes.

5.     Remove some water from the bag by pouring it into a container (not back into the tank). Add water from the tank to the bag again, but this time, add double the amount of water you added in step 4. Wait another five minutes.

6.     Repeat step 5. This gives a total acclimation time of 30 minutes, which is appropriate for all basic freshwater fish purchased at a local pet store. Fish that have been shipped to you by overnight express will require a different acclimation procedure. Check with your dealer for instructions.

7.     Gently release the fish into the aquarium.

Watch all the fish in the aquarium closely for the first day. Try feeding them to see if the new fish are feeding properly. Watch for fighting between the old and new fish. Some chasing almost always occurs, but if any of the fish develop ripped and torn fins, they may need to be isolated.



There is always a danger that a new fish will bring some disease into your tank and infect all of your fish. You can minimize this problem by choosing your fish carefully, but the risk never entirely goes away. Public aquariums and people who have very expensive fish collections (like some marine aquarists have) will quarantine all new fish to reduce this risk even further. This process isolates any new fish for a time so any diseases they may be carrying can be identified and treated. To do this, you’ll need a dedicated quarantine tank, and this is sometimes too costly for beginning home aquarists to manage. Still, having a quarantine tank can be helpful, not only when buying new fish, but for isolating any of your existing fish for one reason or another.


Observing Your Fish

Watching your fish is really why you have an aquarium in the first place—you cannot enjoy your tank if you don’t look at it! Proper observation can also show you impending disease problems, or if some fish are not getting along together. Fish are quite able to see outside their aquarium—they soon learn to beg at the water’s surface for food when they see you enter the room. Because fish will interact with movement outside their tank (and this changes their behavior), it is best to make your observations from a darkened room and with the aquarium light on. Fish cannot see you under these conditions, so their behavior will not include reacting to your presence. It is important to observe your fish every day, not just at feeding time.

Click here to preorder a copy of TFH Publications’ new book, Your First Aquarium.

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