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An overstocked aquarium is an aquarium that either has too many fish, or is too small for the few fish it has, even if that is just one to two large fish.
There are general rules of thumb for stocking both freshwater and salt water tanks, but these rules are impacted by many factors and must be adjusted as such. They are also based on very small fish and don't apply to fish that grow large.
A safe rule of thumb for small-to-medium freshwater fish is 1" (2.54cm) of fish per 2.5 gallons (9.5 liters) of water, and for marine, 1" (2.54cm) of fish per 5 gallons (19 liters). In both cases one should use the adult or eventual length the fish will reach. Not the length when purchased. Otherwise your fish will outgrow your aquarium. A medium fish can be considered up to 4" (10cm) full grown.
Fish that grow very large count exponentially more than fish that remain small. They require more swimming space and better filtration. They produce more waste and are often aggressive if cramped. A deep-bodied 18" (45.7cm) fish, for example, requires a minimum 250 gallon (946l) tank to be able to turn comfortably and have a little swimming space. That's about 14 gallons (53l) per 1" (2.54cm) of fish! By this example you can see how the rule of thumb escalates dramatically for larger fish.
Since surface area affects the oxygen exchange in a tank, tanks that are taller and narrower support fewer fish than tanks that are longer, even if both tanks hold the same amount of water. Also, most fish swim horizontally, not vertically, so width is more desired than height.
Tanks that are overstocked will not only provide a stressful environment for the fish that will likely lead to disease, but overstocked tanks produce a lot more work for the aquarist. Maintenance must be done more often on a tank that is overstocked.
Signs that might point to an overstocked tank are:
If your tank is overstocked there is a way out. Most local fish stores will gladly take fish off your hands for credit if the fish is valuable, or as a courtesy, if not. Either way you win by reducing the population in your tank. Check with your local fish store before bringing in any fish. Or if you really want all the fish you have, you can consider upgrading to a larger tank.
Overstocking is the most common mistake made, and the costliest to fish and aquarist alike. There is nothing more frustrating than trying to care for fish and being unsuccessful for all your efforts. If this sounds like you, your tank may be overstocked. Reduce the population and see how much more rewarding the hobby can be. Your fish will appreciate it too!
If you are running a reef tank you might even want to consider not having any fish at all. Even one fish can add a huge amount of pressure to the reef system, especially in a small tank and an overstocked fish tank can end up killing your corals.
Many successful micro reef aquariums don't have fish, although they do often have smaller marine creatures like shrimp.
If you want fish but can't afford a larger aquarium, consider getting a fish only tank. If you set up an aquarium with fish properly it can take a lot more than a reef aquarium can.
If you have overstocked you might also consider getting a another tank off an auction site or from a second hand store. They can often be picked up very cheaply. It is also not a bad idea to have a second tank handy, as it can act as an extension of the main tank in different ways. You can keep fish in there in order to quarantine them before adding them to the main tank, or use it as a fish hospital if one of your pets is sick.
Or you can test if a new fish is going to be aggressive with less valuable fish, before putting him in with the others.
If you set up your extra tank
to be a sump, it will also help to filter the water in the main tank, and you can disperse additives in there without overloading the fish. Having a sump can increase the number of fish you can stock in your main tank as well.