How Many Fish per Gallon: The Wrong Way to Measure Tank Capacity

A very common question on forums 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 hobby 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 produce as much waste as the 25 neon tetras altogether. I used 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 another 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.

++ Best Filter for Small Tanks

Looking for information about the fish you are keeping is a very important aspect of the hobby. Fish profiles are easy to find online (and we have plenty here in the Aquarium Club).

Water quality is what I believe to be the other very important thing to look at. Assuming you provide the correct maintenance and don’t overfeed, nitrates and phosphate should be within normal ranges. High levels will create stress which often leads to diseases and death. Preferably, those levels should stay below 25 ppm and ideally below 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 an acceptable range, there are probably too many fish in your tank.


3 Responses

  1. I recently retired and have taken up cold water fish keeping gain after a 25 year absence. It is unbelievable how the hobby has changed!

    Some things I want to check with you.
    I used to run a under gravel filter with the normal sponge and pump filter. This meant I replaced 50 % of the water every two months. Any reason I cannot do so again?

    I still have my old water distiller and it is working 100 %. Would it be better if I used distilled water during the water replacements / top ups in stead of tap water with the declorinater liquid added?

    1. Hi Okkie, thanks for your comment. Glad you’ve decided to take up fish keeping again!

      Anyway, to your questions. Under gravel filters are still viable . However, power filters give you superior mechanical and chemical filtration. In the end, it comes down to this. If you liked how your under gravel filter worked before, then I think you should use it again.

      You may wish to switch to a power filter however because it does give superior filtration capabilities. Power filters are also the most common filters in use, so it will be easier to find accessories for and information about power filters.

      Distilled water lacks minerals, so tap water with dechlorinator would be your best choice. Alternatively, you could use special mineral supplements for your distilled water to replenish it’s lost minerals, but it would be simpler to use tap water with dechlorinator.

      Hope that helps!

  2. Hi I have a 25 gallon tall tank with 9 guppies 5 harlequin rosaboras and 4 2” small bottom feeders and 1 fat mouth pleco that stays 3” -4”max and ghost shrimp that are producing fry. And I do often clean tank. I have a Marineland 75 gallon filter on it. I use seachem prime resin bag, seachem ammonia bag white thick filter pads and fine filter pads and the bag off good bacteria in the filter. 1” per gallon rule I have exceeded if my math works! I don’t loose fish and I do clean often like weekly to keep bio under control but what is ideal size for this tank? As guppies I think have a heavy bio load compare to size and seem to be little gangsters compared to rosaboras. I would like my ghost shrimp fry to live and not be guppy food – Should I remove guppies from tank? Thanks for feedback

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