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Notes on Livestocking


Sara Allyn Mavinkurve


As a crew member of Wet Web Media, I see a lot of questions regarding marine aquarium live stocking. These questions usually look like one of the following (in order from my favorite question to my least):

Q1: I want an X and maybe a Y, what kind of system will I need to keep it properly?

Q2: I have a system with X and Y, can I keep a Z?

Q3: I have my tank. I want to put an A, a pair of B, 3 Cs, 2 Fs, an R, and maybe a few Zs. What do you think?

Q4: I just bought a Z, is it going to survive in my system?

Q5: I have A, a pair of Bs, 4 Cs, 3 Fs and an R in my 200 litre tank. My Z just died, what did I do wrong?

My answers to such questions, in brief, look like the following for the corresponding questions:

A1: Good for you

A2: Maybe

A3: Slow down!

A4: You should have asked this before you bought it.

A5: You're an idiot. (Just kidding! I would never actually say that.)

There are a few mistakes most all new aquarists make when setting up a fish only tank. They tend to either not think, nor research, enough about the fish they want to keep. Or, they make a long grocery-list of fish and want to keep them all. Even if you have a huge tank in which you might be able to keep almost every fish in the book, you shouldn't add more than one type of fish at a time, leaving several weeks in between additions. As it's been said many times, and many ways, the key to successful aquarium keeping is patience (and diligent research).

Be Smart about Finances

Just as important as planning out your system as is planning out your finances for the task. I'm always puzzled when someone emails WWM, stating that their fish are dying (or dead), but they can't afford a protein skimmer or any more water pumps. Marine fish are expensive. It makes little financial sense to try and cut corners setting up the system when doing so will almost certainly cost you more in the long run. If you're short on funds, buy gently used equipment. Given that more than half of new marine aquarists don't stay in the hobby even a year, there is a wealth of such second hand supplies out there for pennies on the dollar. You can also save money by only buying what you need. I've seen too many aquarists waste hundreds of dollars on additives they don't need or algae killers that don't really work. Make a list of everything you need and don't purchase any live stock until you have the means and supplies to set up the entire working system. You won't regret it.

Have a 'Focus' Fish

My advice to anyone starting a small to medium sized fish only tank (up to 400 litres) is to think of the fish you want to keep most. Pick one fish that's your absolute favorite. Plan your system around that fish. Then, complete your stocking list to cater to that fish. For example, don't pick fish that will compete too closely with the chosen animal for any limited food source (unless there's more than enough to go around). For those setting up a large tank (>400 litres), think of a type of fish you'd most like to keep (I.e. triggers, tangs, angels, etc) and set up the tank to cater to that type of fish. Taking this approach greatly simplifies your stocking plan. Instead of picking out fish randomly and trying to figure out if they'll all get along, you can have your favorite fish in mind and focus on what other fish might do best in the same system.

Another common error many new (and even seasoned) marine aquarists make is in stocking their tanks to full capacity. Ideally, your tank should be 'under-stocked.' You don't want to push your system to its limits. This leaves little room for error. For example, if you stock your tank its maximum 'bioload,' disaster will strike the day one of the fish dies and it takes you a few days to find the dead animal. Also if you fully stock your system from the start, your fish will inevitable grow and one day your system won't be able to filter the waste from the larger, still growing fish.

So, how do you know how many fish you could safely keep in one aquarium? Calculating what many aquarist's call 'bioload' can be rather tricky. When I set up my first marine aquarium, I was told that I should not have more than '1 inch of fish per gallon of water' (converted to metric, roughly about 2.5 cm of fish per 3.8 liters of water). While such rules are tempting for their simplicity, they don't take into account many important factors determining how much waste and nutrients a system can process. For example, how do you count the 'liters' of the system? Do you use the actual volume size of the tank or volume of water in the tank? The can actually be very different volumes. A significant volume of aquarium water is displaced by sand, live rock, etc. Theoretically, this might decrease your 'liters of water' per which you calculate the centimeters of fish you could keep. However, healthy live rock and a properly maintained sand bed actually add to the biological filtration of your system. This also has its limits though. At some point, additional sand and live rock don't add to filtration enough to justify the water they displace. Unfortunately, these matters themselves could be the subjects of separate articles themselves, but this article is about selecting live stock. So how do you figure out how many fish you can keep? To answer this question, I again suggest that you first pick our absolute must-have fish and figure out the minimum tank size requirement for that fish (the needs of that fish as a full growth adult, even if you purchase it as a juvenile). If you select and add other fish, keeping in mind to carefully keep from interfering with your favorite fish's health and happiness, chances are you will stop adding fish before you hit your maximum bioload. Typically, compatibility issues will arise before those of biological filtration capacity. Fair enough, my 'rule' (though I like to think of it as more of a strategy than a rule) isn't perfect either. You could quite likely keep a school of 20 Anthias in a 200 litre tank without any compatibility issues, but such a tank would very likely be over its filtration capacity. However, I hope the general spirit of what I'm saying isn't lost. Focus on one fish or type of fish and built your design your system and your stocking list around that fish.

Who goes first?

Your focus fish shouldn't necessarily be the first fish you put in the system. Who goes first depends on the temperament and tolerance (or 'hardy-ness') of the fish. Some fish are very sensitive to unstable aquarium conditions and shouldn't be added to a tank that's less than a year old. Marine aquariums, despite what anyone might tell you, actually take several months (if not up to a year) to fully 'mature.' Not only that, if you are new to marine aquariums, it might likely take you at least that long to get the hang of things and to finally stabilize your water conditions. Or, some fish, such as Dragonettes, need a tank that's been well established with benthic life. Such fish are not good first additions to new systems. No fish should be added before the tank has had at least a month to cycle and there are no measurable ammonia or nitrites in the system. Only the most hardy fish should be added within the first two months. Leave at least two weeks in between additions.

Another consideration for ordering addition is aggressiveness. All fish in the tank should be of relatively similar temperament. However, some are bound to be more aggressive than others. Usually, you want to add the less aggressive fish before the more aggressive fish. Another 'trick' some people find success with is to rearrange the rock work every time a new fish is added to the tank. At least in theory, this causes the resident fish to lose their 'territories.' The confused resident fish might be less likely to try and defend a territory they can no longer find.


Compatibility charts only give you a general idea of which fish might (or might not) get along. Much of the actual result of putting two fish within any given system will depend on how large your tank is, how much rockwork you have, the order in which you add the fish, and the individual 'personalities' of the fish. Not all fish of a given species can be expected to behave the same way. As human beings are individuals, to a lesser degree, so are fish. Keep in mind that just because your friend down the street managed to find two wrasses that don't try to kill each other, that doesn't mean you'll be as lucky. Nor does it even mean that if you took those same two fish and put them in your system that they would still get along.

There are things you can do to help keep your fish as 'peaceful' as possible. Feed your fish well. Hungry fish tend to be crabby fish (no pun intended), while fat fish tend to be less so. Of course, this all has to be taken in light of the fish's natural temperament. A clown trigger is going to be a bully no matter how well you feed it. In the case of such highly aggressive fish, about the only way to keep them from bullying every fish in the tank is to put them in as large a tank as you can possibly afford and/or have room for. Extensive rock work can also help since it makes it harder for fish to chase each other.

Additionally, whenever adding a new fish to a system already populated with resident fish, it helps to move as little as half a litre of water from the main tank to the quarantine tank (and from the quarantine tank to the main tank). Do this at least once a day during the last week or two of quarantining the new fish. This gives the fish a chance to 'smell' each other before actually meeting 'in person.' It lessens the shock to the new fish and to the 'surprise' to the resident fish.

Other Compatibility Issues

Fish can be incompatible tankmates for a few reasons, only one of which is temperament. Two fish can be incompatible for having different needs which can't both be met within the same system. For example, some fish benefit from extensive rockwork while others do better with plenty of open space or an open sand bed. The intense lighting needed for a reef tank can blind lionfish. All of these types of issues must be thought about in advance of any livestock purchase, even before the system is set up.

Always remember that patience, planning and careful researching are the keys to many happy fish and rewarding days of pet fish keeping.


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