We've all heard some of the general “rules of thumb” for stocking an aquarium: an inch of fish per gallon, one fish per gallon, more fish = less aggression and countless others. So, what works best? There are several approaches that can help make stocking an aquarium easier.

Research, Research, Research!!

It's no secret that I for one am a big fan of research. I'm not talking in-depth taxonomic studies or long term observational studies, just a little planning and reading up on what you would like to keep. We get a lot of questions like “I set up a tank, now what can I put in it?” or “I bought this pretty yellow fish, what can go with it?”. You may be able to save yourself some time, frustration and possibly disappointment by planning out what you might like in your tank before setting it up or buying any fish for it and having an idea of what you would like to put in it down the road. That way, you will be able to decorate and filter your tank in the best way for what you'd like to keep and you'll avoid getting those impulse-buy tankmates that eliminate any chance of having that special fish you've had your eye on for months.<

Along these same lines, be sure you are aware of the behavior of each item you choose and its requirements. There is nothing worse than bringing a fish home, only to learn that you have to spend more time preparing its food than your own or finding out that it is a known predator of that favorite fish you already had. A few basic aquarium fish books can help a lot with the basics and can even help you find fish you never even knew existed.

An inch per gallon? Only sometimes.

This is one of the most common stocking “rules” we hear but is often misunderstood and misused. Let's compare a few common groups of freshwater fish: tetras, goldfish and cichlids. First, the “inch per gallon” or “fish per gallon” rules have to take the adult size of the fish into consideration. Sure, a fish might be one inch when you buy it, but if that fish grows into a footlong adult? That changes things. Then, what about the body mass of the fish and the waste it produces? Six little one-inch-long tetras will certainly affect an aquarium differently than a six-inch-long goldfish. Goldfish just produce a whole lot more waste, eat a lot more and have a lot more mass and size behind that six-inch-length than all of those little tetras put together.

Next, what about the behavior? That same six-inch goldfish is going to have a far different temperament than, say, a six-inch Green Terror cichlid. Two six-inch goldfish could be perfectly happen in a well-filtered 45 gallon aquarium; the two Green Terrors may well try to kill each other. So do we through out the “inch per gallon” rule altogether? Not necessarily. It is fine to use as a very general and basic guideline for small fish like tetras, danios or livebearers if you take their adult size into consideration, but don't take it as gospel if you are keeping anything larger.

Zone Defense: It works in sports, it works in aquariums.

When stocking your tank, keep in mind that all of the fish won't be spending all of their time in the same area of the tank. Looking at your aquarium from the front, you can divide it horizontally into 4 zones. The middle two zones are where a lot of fish hang out. In freshwater, this is where you'd find tetras, angelfish, barbs, and some cichlids. In saltwater, this would be your tangs, damsels, clownfish, and some groups of wrasses. The top quarter section is more of the top dwelling fish. In freshwater, these are fish like hatchets, killifish, rasboras, and mollies and in saltwater, this would be some cardinals and dartfish gobies – fish whose mouth are more on the tops of their heads and point upward for those prey items on the surface. The last section, the one on the bottom of the tank, is home to the bottom feeders - catfish, blennies, loaches, and gobies. Some saltwater fish and freshwater cichlids that spend a great deal of their time in and around rockwork also would count towards this group.

Stocking an aquarium with all three sections – top, middle, and bottom – in mind will help you make the most of the space you have. Instead of having a lot of fish that hang out in one of these zones, choosing fish from all three can give you a more complete look to your tank and can help spread out the activity and aggression throughout the whole tank.

The More, The Merrier? Or One Big Spotlight?

There are two big ways to plan an aquarium – having an active tank with lots of activity and schools of little fish, or have a showcase item like a big saltwater angelfish or showing cichlid and build the rest of the tank around it. Both can be stunning in their own way but take some planning. Having a tank full of different schools of little fish can be interesting and active. You can have a higher number of fish this way but there isn't one big thing to focus on. Or, you can have one big fish like a Discus and complement it with just a few other little fish for some subtle activity and to help with clean-up. Some of the most interesting tanks can be species-only tanks. These tanks have just one kind of fish like an aggressive cichlid or a saltwater oddity like a frogfish, and nothing else. These are the true “pet fish”. They can be fascinating but aren't as diverse as community aquariums.

Biotopes – A Little Piece of Nature

Some of my favorite aquarium have been biotopes. These are tiny pieces of a specific environment where everything in the aquarium is designed and chosen around that location or habitat. For examples, a Caribbean biotopes would have only fish and invertebrates chosen from the Caribbean, so a fish from the Indo-Pacific would not be chosen for this tank. This is how most public aquariums are designed and is one of the more collector's approaches to aquariums. Instead of relying on the impulse buy, this method is all about planning and choosing the perfect addition for your little slice of nature. Much better than a postcard for remembering that trip to Hawaii!

These are just a few approaches to choosing the next addition to your favorite aquarium. Feel free to share your own and never be afraid to ask if you aren't sure if what you want is right for you!