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How Many Fish Can I Put In My Tank?

Too Many Fish

This is probably the third question on a new aquarium owner’s mind, right after where they will put it, and how big it will be.

“How Many Fish Can I Put In My Tank?”

Aquarium technology and knowledge have grown past the old “one inch of fish per gallon” rule.

We have had greater success by building happy and healthy aquarium communities, rather than trying to reach a number.

We consider factors such as the capacity of the life support system, the “swimability” of the aquarium, and the number of available “homes” and “zones” in the aquarium.


An Aquarium’s Life Support Capacity

Probably the most important part of the answer to “How many fish can I put in my tank?” has to do with how many it’s life support system can keep alive and healthy.

An aquarium’s life support system removes toxins and other pollutants from the water. The more robust ones replenish the oxygen and add other life-sustaining elements and materials. Its capacity depends on how much organic waste can be quickly converted into less poisonous materials, which in turn, depends on how much waste is being produced.

Adding too many fish at one time can out-pace the bacteria which converts the waste. So can adding fewer fish which produce more waste per fish. More waste means increased demand on the bacteria, and it may need to grow more bacteria to convert the waste quickly enough. Otherwise, an aquarium can have the toxins Ammonia and Nitrite waiting to be processed, and possibly affecting the fish in the meantime.



How much room is available for the fish swim in? Do they have room to swim freely? Or are they bumping into other fish and having to change their course all the time?

This is especially important for tangs and angels. Some fish stay within a small area, and others need more free space to swim in.

Swimability refers to both the open spaces and the closed spaces. Good aquascaping allows for plenty of swimming and holes which the fish can call home.


The Number of “Homes” (or “Hiding Places)

Fish generally do three things: eat, defend territory, and make more fishes.

When there are more fishes than there are hiding places and places to call “home,” it can increase their stress level, which affects fish greatly.

Fish build a pecking order in their community. The more quickly this is established, the better everyone thrives. The hiding places and holes are important parts of this process. Fish need a place to get away from the bullying and posturing, and to be able to make into their own home in the tank.

Here’s a secret: In my experience, larger tanks with many hiding places tend to help keep fish in an aquarium which usually “shouldn’t” be in a tank together. It doesn’t always work out, but I have seen that the tank owners with aquascaping that builds in a large selection of “homes” often have more success in pushing the envelope of fish selection.



This is one of those lesser known secrets that lets you have a higher number to the question of “How many fish can I put in my tank?”.

Different fish live  in different zones of an aquarium. Some fish live high in the open water spaces. Others live within and among the rocks. Other fish, still, are burrowers, living in the sand, underneath the rock or other objects.

Understanding how to balance this out lets you have more fish, without the competition for space. So they live together peacefully and healthfully. This takes a bit of planning and knowing your fish, and ReefTechs Aquarium Consultants can help guide you through the process.

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