How many fish can your fish tank hold comfortably? It’s the age old question that every aquarist wants a quick answer to. Most new aquarist just want a rule of thumb. If you have been in the hobby you want to make sure you are not overcrowding your tank if you have had problems. The golden rule used to answer this question most often might be bad advice if it’s not completely understood. Because of this, let me suggest a better way.
One Inch per Gallon of Water – The Golden Rule
The most common advice that you will hear for stocking your fish tank is to add no more than 1 inch of fish per gallon of water to your fish tank. The problem is that the answer to this question is not really as simple as this rule by itself. There are many other factors that influence whether or not this rule really works.
To help illustrate the point, think of a 12 inch Oscar as opposed 8, 1½ inch Neon Tetras. They both add up to 12 inches, but the room they actually take up in your tank would be dramatically different. This shape difference also has a big effect on the waste output of your fish.
For smaller fish species, this rule works pretty well. For larger, it does not. Also, don’t forget that the rule is referring to the full adult size of your fish.
Then It Must Be Surface Area Rule
The surface area calculation is pretty simple as well although it’s not perfect either. Basically you multiply the length times the width of your tank. This gives you your tanks surface area. Under this rule your tank would house 1 inch of skinny fish for every 12 inches of surface area. Your tank would also house 1 inch of full bodied fish for every 20 inches of surface area.
You might have already figured out that the surface area calculation does not take into account how tall your tank is. That is because it us based on the amount of oxygen exchanged. This only happens on the surface of your tank of course.
So What Do You do?
I would suggest using the 1 inch per gallon rule with one modification. For smaller fish, use the 1 inch per gallon calculation as it is. For larger, fuller bodied fish, you’ll want to use a multiplier of 3 and count 1 inch of fish for every 3 gallons of water. For medium size fish use a multiplier in between like 1 inch of fish per 2 gallons of water. With this method you can take into account all sizes and shapes of fish.
The oxygen exchange consideration in the surface area method is not that important for the most commonly sold fish tanks. Most fish tanks sold retail come in pretty standard shapes. Generally, they will have adequate oxygen exchange whether you have a tall show tank or a longer standard tank shape. You should however, consider the surface area rule if you do by chance have a very odd shaped fish tank with very little surface area.