How Many Fish Per Gallon?

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One of the most important factors that comes into keeping fish in tanks or ponds is how many fish you can get away with keeping in one system. The tried and true rule of how many fish per gallon is: 1″ of fish per gallon of water. 

Well, that’s great and all, but have you ever considered what 1″ of fish actually entails? Is 1″ of neon tetra the same as 1″ of koi? Taking into consideration species different physiology and energy conversion, there is almost no way this rule can hold up. It is a dumb, outdated rule and should be thrown out.

If we can’t use “1” of fish per gallon of water, then how many fish per gallon should we have?

Well, it will depend entirely on the particular species you wish to keep. This decision should be made prior to purchasing any fish supplies. If you want more than one species in a system, make sure they get along first and won’t eat each other. Do they have the same diet? Are their water quality tolerances within the same range? What size tank can you accommodate? Is it big enough for all the fish to feel safe and comfortable?

All fish are different in their body type, energy conversion and territoriality. All of these characteristics can be manipulated by nutrition and water quality. Proper nutrition and excellent water quality will result in fast growth through efficient conversion of food to muscle and fat, although fish that are adequately fed will often run out of space and will feel the need to defend their space. It is a delicate balance to pursue.

One must also take into consideration how big their fish might get. Consider the koi. Although they start as small as a goldfish, they can grow up to 2′ or longer! By keeping koi in an improper size container, you may actually stunt them and keep them from properly developing. Growing to the size of their container” is also a complete myth. If you keep feeding them a high quality diet, they will keep growing and eventually turn into a C- or S-shaped fish. Even just given the basic cyprinids, koi and goldfish, it is hard to guess how big your fish will get.

It is always best to plan for the biggest your fish can grow in order to determine how many fish per gallon your system can hold.

The fish will have enough room to stretch out and your filtration will be more than you need (which is always a good idea). Even goldfish can grow to over 12″ long. We had one patient that needed his own 100 gallon tank; he was just that big! Sure, they may start out all cute and little, but I guarantee they will grow. Koi require at least 250 gallons PER FISH! It might sound silly when they’re cute and little, but they have the potential to grow VERY large.

So, what is a fish owner to do? Well, I wish there was a straightforward rule of thumb, but there are just too many species differences to consider. Try to follow the following guidelines:

  1. Research your intended fish pets before you buy anything.
  2. If you are mixing species, be sure you are aware of their territory requirements. Look here and here for some good species-specific info.
  3. If you are building your own pond, make it bigger than you want with more filtration than you need.
  4. Get expert advice when mixing species or getting started. If not your local fish veterinarian, visit a local pet store that has permanent setups (not large, divided wall units) similar to what you are looking for.
  5. When in doubt, go bigger.

Here are some examples:

How many fish per gallon can you fit in a 20 gallon tank?

This 20 gallon goldfish tank contains 3 goldfish and one long-fin zebra fish who thinks he’s a goldfish. Since this picture, they have gained 1 fancy goldfish friend. At this current size, there is plenty of room for everyone. Since this tank is in our office, we carefully watch their diet and make sure they receive a high-quality diet, but not enough calories to make them grow quickly. They will eventually need to move into one of our larger systems. Several of their friends live in our 500 gallon goldfish pond!

However, this is also a goldfish tank. This fish was a little snug in his 75 gallon tank, so he got upgraded to a 100 gallon! This is a very big goldfish and all comet goldfish have the potential to get to this size. Fancy goldfish, not so much, but they have other issues. A few years ago, due to some problems with water quality, his growth had stopped. Fix the water, give him some more space and a better diet, and he started to grow again!

These 5 koi are in a 5,000 temporary pond. They were upgraded to a 9,000 gallon pond about a year ago and gained 4 friends of similar size. The two white and orange fish won the Grand Champion and Reserve Champion at the ZNA NorCal Koi show this past April. They are both almost 30″ long and weigh over 20 pounds. They need lots of room to swim to produce lots of muscle and have fancy diets that keep them big and strong.

And these are gouramis. We have had several batches in our hospital in quarantine for a private aquarium breeder. We have found that these guys do not like being in close proximity and like lots of plants to hide in. These 12 fish had a 40 gallon tank to themselves with 3 large plants. With a larger tank and more room to hide, these guys had considerably fewer disease issues and maintained their body condition significantly better.

Hopefully these examples show the many issues with sticking to a clear rule for # of fish per gallon of water. If you have any questions, feel free to comment or contact our office directly.

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3 thoughts on “How Many Fish Per Gallon?”

  1. Pingback: A Guide to Buying New Fish – Aquatic Veterinary Services

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  3. Pingback: How to Be a Better Fish Parent - Aquatic Veterinary Services

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