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Discussion Starter · #1 · (Edited)
When it comes to marine aquariums one of the most frustrating things can be the conflicting advise given on the Internet, in books, at at the pet shop as to what fish will do well in a particular aquarium environment. It seems that everything is up for debate, from which fish mix well together, to what size tank a fish can live in, and even if a fish is compatible in a reef environment. Although I can not resolve all of these issues in a short article, nor am I an authority on the subject anyhow, I do think I can help you work through some of the more common issues that you will face in creating a stocking list for your aquarium.

The first question many people ask when setting up a marine tank is "How many fish?". My first response to such a question is that it depends on the exact fish you select. This has very little to do with size, and everything to do with BEHAVIOR. Understanding fish behavior inside the aquarium is the key to creating a list of fish that will get along together, and allows you to understand the correct order in which the fish should be added.

Aquariums under 6' in length.

Lets look at a few examples to help clear things up. One of the most common aquarium sizes for the new marine hobbyist is the 29 gallon tank. When thinking of a 29 gallon tank, think of the physical environment available for the fish. The tank is 30'' in length, which tells us a lot about the amount of swimming space and territory that will be available for the fish. Such a tank does allow for some advantages, such as the reef structure. A 29 gallon tank is large enough that you are able to build a nice reef, using dry rock and live rock. In small tanks such as this you will want to add MORE rock per gallon than larger tanks. Adding more rock (less water) will actually create MORE space for the fish to swim and call home, because of the BEHAVIOR of the fish you will be looking to keep in this tank. I would suggest than any tank under 6' in length should be very crowded with rock forming a massive reef structure (or "rock structure" if it is a fish only tank). Lets move on and look at some fish to explain this in more detail.

It is easy to eliminate fish that should not be considered for a 29 gallon tank environment. For example, you won't be able to keep fish which are open water swimmers, such as Tangs, Rabbitfish, and Butterflyfish. These fish occupy large areas of the reef in the ocean and will never adapt to such a small environment. In fact, many of these fish are rather peaceful in the proper tank size, but become extremely aggressive when kept in tanks of less than 6' in length. It should also be obvious that you will not be able to keep fish which grow large in size, such as Pufferfish, Triggers, Lionfish, Groupers, and Lunar Wrasse. When you start to eliminate fish that can not thrive in smaller tanks, it becomes easier to categorize fish that can be kept.

Looking at fish that are appropriate for a tank of under 4' in length, such as a 29 gallon tank, you will see that they all have one thing in common. These fish are rock dwelling species, meaning that they spend most of their day very close to the reef, never venturing far from the small nook that they call home, or swimming in and out of the rock structure that makes up the reef. Lets call this "Group A". Examples of these fish include Basslets, Pseudochromis, Gobies, and Jawfish. You could also consider fish that stay small in size and generally dart in and out of the reef area, or swim about feeding on the microfauna. These fish are active fish, but not open water fish, and as such do not require as large of an aquarium. Lets call these "Group B". Examples of these fish include Dwarf Angelfish of the Centropyge genus (which do well in pairs), Hawkfish, Firefish (best in pairs), Cardinalfish, Blennies, Chromis (best kept in pairs not schools), Anthias, Carpenter Wrasse, Fairy Wrasse, and Clownfish (pairs).

As you look at the list above, think about the physical design of your tank. The numbers of fish that you keep will depend on how much rock you have and how well designed your reef is. Fish need an area to consider home, especially smaller fish such as those we are looking at here, and their home needs to be well defined. Each fish needs a place to settle down at night to sleep, and it needs to feel protected in this physical environment. The more rock that makes up your reef, the easier it is for fish to define their territory, enabling you to keep more fish in these sized aquariums.

Hopefully at this point you are starting to realize that you can more easily keep large numbers of fish from group A than you can group B. Fish in group A are going to stay in a more confined area, rarely venturing more than a few inches from their home. With a large reef structure you can easily fit 4 or 5 of these types of fish in a tank under 4' in length. You will be able to judge this based on the BEHAVIOR the fish exhibit when you add them to the tank.

Looking at group B you should realize that these fish are going to be out in the open more, and as such much more visible to the other fish in the tank. Keeping larger numbers of these fish becomes more difficult. When adding these fish to the tank, you also need to consider adult size. That being said, we all hate generalities when it comes to stocking an aquarium, but without them you can't make many decisions on your own. For this reason I am going to provide you with some guidelines as we go along.

So, rule of thumb #1: For tanks under 6' in length the adult size of a fish times 10 should not exceed the length of the tank. Using this rule you are able to reduce compatability mistakes. For example, in a 29 gallon tank of 30'' in length, you should not purchase a fish with an adult size of greater than 3''. This allows you to properly select members of the Centropyge angelfish that will fit your tank. You can easily see that a Bicolor Angel is not a good fit for a 29 gallon aquarium. You should instead consider choosing between the Flame Angel, Pygmy Angel, Flameback Angel, and African Pygmy Angel. Each of these fish stay small and will thrive in this size aquarium.

There are other considerations that you will have to pick up on as you go along. For example, you may have noticed that Clownfish and Centropyge Angelfish do very well in pairs. Although this is true, you have to be very careful with mixing fish within the same genus into the same aquarium, unless they are the same species and are added at the same time. This leads us to rule of thumb #2: In an aquarium under 6' in length, do not add more than 1 member of the same genus to the aquarium, unless they are of the same species, compatible as a pair, and added at the same time.

The final consideration is space. Even the largest reef structure will be limited by the size of the aquarium, so let me give you one more concept to consider. Rule of thumb #3: For an aquarium under 6' in length, consider adding 2 members of group A for every 15 gallons of tank, and 1 species of Group B for every 15 gallons of tank. This means a 10 gallon tank would get 1 fish from Group A and one species from Group B. A 20 gallon gets 2 fish from Group A and 1 species from Group B. A 29 gallon gets 4 fish from Group A and 2 species from Group B. This should help to give you an idea of how these fish will behave in terms of territorial behavior and compatibility. Note, a "species" may consist of 2 fish of the same species. In other words, a 20 gallon tank could probably house 2 Flameback Angelfish successfully, along with 2 fish from Group A.

Using these guidelines, lets look at a possible stocking selection for a 29 gallon tank with a large reef structure. In this example you could add 4 fish from Group A and 2 species from Group B. As such, you might consider 1 BiColor Blenny, 1 Royal Gramma, 1 Diamond Goby, 1 Jawfish of choice, 1 Flame Hawkfish, and a pair of Flame Angelfish. {edit: It has been pointed out that a BiColor Blenny and Royal Gramma will often fight due to similar color patterns. I agree with this point, but rather than change the article, I thought I would add this comment as an indication of how complicated these situations can sometimes be. Similar color patterns also need to be avoided.} The above example gives a total of 7 fish in a 29 gallon tank, which is far greater than most people would ever consider. Yet the stocking list is very organized and has very little risk of failure. In this example if you wanted, you could eliminate the pair of Flame Angelfish and instead keep a pair of Ocellaris Clownfish. Or you might decide to keep 1 Flame Angel and 2 Ocellaris Clowns. There are choices to be made, but you do have choices and can have a great mixed community of fish.

To be continued...

4,122 Posts
Discussion Starter · #2 · (Edited)
Aquariums of 6' in length or greater.

When you begin to move to larger sized aquariums things start to change. The length of the tank allows for the rules above to begin to bend. If you have not read the section above, I highly suggest reading it first. Even if your tank is not under 6' in length, the ideas presented above should help you to understand fish behavior and make it easier for you to stock a larger aquarium successfully.

When looking at a 6' tank you begin to have almost all fish as possible choices in the aquarium. Granted, there are some fish that should never be kept in a home aquarium, such as the Unicorn Tang, Red Snapper, Panther Grouper, etc. These fish grow far to large for the typical home hobbyist, so unless you have an aquarium of nearly 1,000 gallons in size, you should not be considering such fish. But for the rest of us, we can be happy to know that most fish we see available at the LFS are going to fit comfortably inside our 6' tank.

An aquarium of 6' in length allows us to also consider Tangs, Large Angelfish, Pufferfish, Triggerfish, Rabbitfish, Lunar Wrasse, Lionfish, Butterflyfish, and Squirrelfish as possible fish selections. Lets call this Group C. The difficulty in stocking from Group C lies more in the stocking order, as opposed to the selection of fish. But before we discuss stocking order let us first modify the rules above for fish selections in these larger aquariums.

Rule of thumb #4: For tanks of 6' in length or more, the "length+width" of the tank should be 5 times the adult size of the fish AND the adult size of the fish should be LESS than the width of the aquarium.

This is a generous guideline, but it does begin to help you distinguish between what fish will fit comfortably into a 125 gallon tank as compared to a 180 gallon tank. For example, looking at a Naso Tang with an adult length of 18'', you can see that this is not a good fish choice for a 125 gallon tank. By the rule, the Naso would require 7.5 feet of total "length+width", which the 125 provides. However, the 125 gallon tank is only 18'' wide, which does not exceed the adult size of the Naso Tang. By the rule, you would need a 180 gallon tank to accommodate the Naso Tang.

Rule of thumb #5: When adding members of Group C of the same genus to the aquarium, add them at the same time. More often than not this will work to prevent you from making a compatibility mistakes. Members of Groups A & B discussed above will generally not have to be added at the same time, primarily because of the added space you have available when dealing with tanks of 6' in length or greater.

Rule of thumb #6: For aquariums of 6' in length or more, you can add 1 unit of fish per 10 gallons of tank size. A unit is defined as 2 members of Group A or 1 species of Group B, with each fish from Group C counting as 2 units. For the purpose of tanks 6' and larger, include Chromis, Damsels, Dragonettes, and the Six Line Wrasse to be included in Group B.

So, lets look at an example. Lets say you have a 125 gallon tank. Your stocking list allows for 12 units of fish. Lets say you begin by adding 2 Percula Clowns (GpB), 2 Flame Angels (GpB), and 1 Coral Beauty Angel (GpB). This is 3 units. You have room for 9 more units of fish. You then add 1 Mandarine Goby (GpB), 1 Diamond Watchman Goby (GpA), 1 Royal Gramma (GpA), and 1 Sailfin Blenny (GpB). This is an additional 3 units. You have room for 6 more units, so you add 1 Yellow Tang and 1 Sailfin Tang to the tank at the same time, both being from the Zebrasoma Genus you don't want to break Rule #5! You still have room for 2 more units of fish!

{note added: I should make it clear here that this rule if meant to apply to a mixed community. You will have to use a bit of common sense based on adult fish size, especially if you venture into Large Angels, the Naso genus of Tangs, and Triggerfish. In fact, you may want to consider each of these fish to account for 3 units each!}

You can see how applying these rules of thumb allows you much greater flexibility in stocking your tank. It is very important to realize that this discussion has nothing to do with BIOLOAD. It has everything to do with fish behavior, aggression, and compatibility. I know what somebody is going to say, and I agree. This is not a perfect system. But it is a system that is far more effective than just putting random lists together and then asking some guy or gal at the LFS if you can have a given fish. It also allows you to post intelligent threads in the forum pertaining to stocking of an aquarium. There are other factors at play, such as fish aggression, but for the most part this method works well.

Stocking Order

As I mentioned before, the difficulty in stocking an aquarium is often in determining the order in which to add fish. Suffice to say, a Yellow Tang and a Flame Angel are compatible in a 125 gallon tank. However, it would be wise to add the Flame Angel first to improve your chances of success. These are things you just kind of learn after a number of years in this hobby. To make this easier, I will try to give you some guidelines on stocking order. There is nothing fancy here, just some generalities that I have picked up on over the years that you may find helpful. I will just list them out below.

1) It is best to add fish to the tank in Group order. In other words, Group A should be placed in a tank before Group B, and so on.
Exception: When adding angelfish, it should not matter which genus is added first to the tank. You can have a Large Angel in the tank before a Dwarf Angel, or visa versa.

2) When putting multiple species of Tangs in the same aquarium, add them according to genus. Ctenochaetus should be added first, followed by Acanthurus (Paracanthurs), and then Zebrasoma. Rabbitfish should be treated as Acanthurus Tangs for the purpose of stocking order.

3) Triggerfish and Large Angelfsh are best as the last addition to a tank. They become very territorial when established and may kill all future additions.
Exception: The Majestic Angel is generally peaceful fish, as are Angelfish of the Genicanthus genus. These can be added prior to other fish without causing harm to future additions. The Niger Trigger is also a frequent exception.

4) When making exceptions to these guidelines, consider diet. Algae grazers will rarely show any concern when adding a predator to the tank, and visa versa.

5) Fish with similar diets and similar color should be added at the same time, regardless of the rules. For example, a Lemon Peel Angel and Yellow Tang should only be kept in the same aquarium if added simultaneously.

6) Large Angelfish with near identical juvenile color patterns should not be placed in the same tank together. For example, the Majestic Angel and Blue Ring Angel are not good tankmates.

I could probably go on and on with these sort of guidelines, but I think these are the most common issues we face as aquarists when creating a stocking order or looking to add a new fish to an established tank.

You may have noticed that I nearly skipped the discussion of a 75-90 gallon tank. I did this very intentionally, as these tanks are very difficult to predict. You could include almost any tank of similar dimensions in this conversation, such as extra high tanks and similar gallon bowfront tanks. These are large enough that stocking from Groups A & B is very predictable. However, with the right mix of fish you can sometimes move into Group C. You can apply Rule #4 to a 75 gallon tank and often be successful, but it takes an experienced hand. For example, a Ctenochaetus Tang will generally work ok in a 75 or 90 gallon tank. You will notice that this genus of Tang is very passive compared to the others of the family, which is why it often works. If you do bend the rules for these tank sizes, pay very careful attention to the rules above concerning stocking order, as you will very rarely get away with any mistakes in a 4' tank.

I hope this helps. I know it is a long article. I also know I really put myself out there for criticism and room for disagreement. This isn't intended to be the authority on the topic of stocking aquariums. But it is a good look at the thought process that I personally go through when looking at compatibility and stocking lists for various sized aquariums.

Good luck & Happy Fishkeeping!

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