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What are Brown distom algae? Why do they grow in our aquarium and how to get rid of them. In this post you will find valuable information about this algae and how to control them.

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Home » Fishkeeping basis

How Many Fish An Aquarium Can Hold?

Submitted by on March 11, 2010 – 9:14 am2 Comments

overstocked aquariumA very common question on forums and Yahoo Answers is how many fish can we stock in a freshwater tank. We know that overstocking our tank will cause problems and we want to avoid it.

There is a weird rule of thumb in the hobbys saying that you can keep one 1 inch of fish per gallon of water. This rule worth nothing and should be ignored for one simple reason: every fish species have a different body mass and produce a different amount of waste. According to this rule, my 25 gallon aquarium could hold a maximum of 25 neon tetras (one inch each) or 12 goldfish (two inches each). Unfortunately, a single 2 inches goldfish will probably produces as much waste as the 25 neon tetras all together.
I use to keep 40 neon tetras in a heavy planted 25 gallon tank without problems. Things could have been different with another fish species.

There are two good ways to know how many fish an aquarium can hold. First, each fish species have its requirements in terms of space. Some will need plenty of places to swim around while others don’t need much. Aggression is an other very important factor to consider. When packed into a small tank with not enough hiding places, some fish will become territorial and aggressive which will create lots of stress. Looking for information about the fish you are keeping is a very important aspect of the hobby. Fish profiles are easy to find online.

Water quality is what I believe to be the other very important thing to look at. Assuming your you provide the correct maintenance and don’t overfeed, nitrates and phosphate should be within normal ranges. High levels will create stress which often lead to diseases and death. Preferably, those levels should stay bellow 25 ppm, and ideally bellow 10 ppm. Those are my personal preferences only. If you do regular water changes and don’t overfeed but still can’t keep your nitrate and phosphate levels within acceptable range, there are probably too many fish in your tank.

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  • Oliver says:

    Useful thoughts on the amounts of fish that are practical for a tank.. What are your views on thin…”on-wall” aquariums? Does this change the rule at all?

  • Hi Olivier,
    IMO, the shape of the tank doesn’t change anything. However I assume it must be difficult to create hiding places for the fish in a tank like that. Maybe those tank aren’t the best for aggressive species.

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