One of the most common questions after cycling a new tank is, without a doubt “How many fish can I put in?”. Since sadly aquariums don’t come with stocking charts attached, aquarists have developed a variety of methods to estimate the optimal number of fish for a given aquarium. An overcrowded aquarium is usually too busy to be aesthetically pleasant, and the living conditions of the fish can suffer unless you are using top-end filters and maintenance hardware, so being picky about the fish you stock is often the best strategy.
The Fish-Inch Per Gallon Method
One of the most common rules for stocking a fish tank is “Allow one gallon of water per inch of fully grown up fish”. This rule provides a decent but pretty rough estimate of how many fish can your aquarium maintain but it is not perfect.
Bigger fish (not only longer but also wider and bulkier fish) take up more space, and produce more waste. This means that 10 inches of neon tetras won’t be nearly as taxing on an aquarium filtering system as 10 inches of Goldfish. Most people also forget that the fish you buy at a fish store are often juveniles, and some can grow considerably bigger.
If you are using this method to get an idea of how many fish per gallon you should stock always use the adult size of the fish and make sure to be reasonable about the space required by these fish in order to swim and be comfortable. Territorial fish may require considerably more water than shoaling species, for example.
Also, keep in mind that not all the space in your brand new aquarium will be water. Rocks, sand and decorations can take a sizeable chunk out of your estimated water gallons, and while stones give territorial fish a nice place to hide, they will still reduce the volume of water. In case of doubt, research your fish before purchase and be conservative, you can always add more fish later on if your tank feels too empty.
Surface Area And The Gallon-Per-Inch Rule
Aquariums used to be pretty standard, but nowadays they come in a large variety of shapes: tall aquariums, classic long-form rectangles, cubes, panoramic and triangular ones… Which means the actual water surface of two aquariums can be very different even if they hold the same amount of water. The less water surface, the more difficult it is for oxygen to enter the water, and as such the more difficult it is to keep large amount of fish.
If you have a unusually shaped aquarium, you can estimate about 12 square inches of water surface for each inch of fully grown up fish. In order to know how many fish per gallon you can stock on an aquarium, you can combine both rules to get an estimate.
The number of fish per gallon is heavily affected by how good your filtering system is. A high-end, highly performing filtering system will be able to adequately deal with the biological load of a heavily stocked aquarium, whereas a low intensity filtering system (such as an under-substrate filtering method) may just not be enough. If you are wondering how the fish stores manage to have many fish in what seems a tiny display aquarium, the reason is that there is a big filtering system plumbed into the small display aquariums which keeps the water clean and the fish comfortable.
Stocking An Aquarium Safely
If you are worried about overstocking your aquarium and losing fish, you shouldn’t be looking at how many fish per gallon you can stock. Instead, make a plan to stock your aquarium slowly, adding just a few fish every week or two. This also means the biological filtering in your aquarium (and the reason why you cycled it in the first place) has a chance to adapt to the new fish. Never add more than 25% of your planned stock at the same time, so the internal ecosystem in your fish tank can adjust.
When planning stock levels, it is worth keeping in mind that some species are more territorial than others, and some fish need to be in groups. For example, having just a couple of shoaling fish will stress them out and make them unhappy, whereas adding two betta fish to the same tank can end up in tears. So any blanket rules about how many fish per gallon need to be taken with a pinch of salt, and used only as a starting point. In case of doubt, less is more.