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.
What experience have you had with choosing how many fish to add you to your tank?
6 responses so far ↓
1 Gazali // Nov 10, 2007 at 6:58 am
The rule is utterly nonsense.
I have a 3 ft tank. My fishes are 7 Blood Parrots, 3 Red-tail Albino Tin foil barb, 2 Apollo Sharks, 5 Clown loaches, 1 Clown Knifefish and 1 Amored Birchir. As you can see plainly, the rule cannot apply here. Yet the fishes are thriving.
There are 3 things that limit the fish population.
1. Dissolved Oxygen Level
2. Cardon Dioxide Level, and,
3. Ammonia.
I do not use an air pump. Instead I use a canister filter with the spray bar arrange to agitate the surface of the water. This is where the gas exchange takes place. Oxygen in and carbon dioxide out.
How do I know if there is not enought dissolve oxygen in the first place? Its when the clown loaches (normally they are at the bottom) starts going up and also the rest of the fish.
On the downside, I have to top up about 2 liters of water every 2 or 3 days.
For ammonia it isn’t a real problem when the tank is fully cycled. My ammonia level is normally zero .
My fishes are big and messy and I feed them pellets, flakes, feeder fishes like catfish, guppies, golfish and minnows and frogs too.
I do weekly water changes of 36 litres every week.
2 Eric // Nov 10, 2007 at 2:38 pm
Thanks for your input Gazali! A great example of the “Rules” being tough, if not impossible to apply in some cases.
3 Aaron // Feb 10, 2008 at 11:44 pm
Hi my name is Aaron and I found that I was able to keep a large amount (by volume) of fish in a small tank- I have a Black skirt Tetra, two Zebra Danios, Two Oto Cats, and a Cory Cat. These are all in a 2.5 gallon tank and haven’t had any problems thus far- not unless you consider algae a spawning problem of too many fish. I do realize that it is a small tank for that many fish, but rest easy- I am in the process of putting them all in a ten gallon tank.
4 Cook.MN // Apr 14, 2008 at 7:42 pm
You realize Gazali that those Clown Loaches grow to over a foot and are best suited for tanks that have a 6 ft. foot print?
5 Hevach // Apr 19, 2008 at 12:36 am
I started my first tank without much consideration about rules and without knowing what I was getting into. I overstocked, as well as stocked too quickly, and to make matters worse, overfed.
With some professional advice and extra work, I managed to get through it and keep it going. I’ve also been extra vigilant for signs of illness, since ick can spread fast enough in a less stocked tank.
The fish are all small (Only a few over 4 inches and none over 5) tropical species, and are fed a diverse diet.
Overfiltering (my filter is rated for up to 110 gallons, while the tank is only 30), using live plants, and more frequent water changes have all helped considerably. Reducing feeding was the biggest change.
Like Gazali, I watch my bottom feeders as a warning bell: As long as they’re at the bottom and not stressing, I know things are under control.
There’s added work and expense maintaining the tank, but disaster is avoidable. I’ve found the extra work quite rewarding, and there’s no denying that the result is very attractive.
The one thing to make especially sure of, though, if you’re overstocking, don’t add overfeeding to that. Overfeeding can cause problems in any tank, but the more fish, the worse it can be.
6 Eric // Apr 23, 2008 at 6:08 am
Thanks for sharing Hevach! Your theme here is great advice. Much easier to time into learning and maintenance rather than fixing problems.
Leave a Comment