Angelfish. Big, intelligent tropical fish that shine with personality. But did you know that angelfish can also be quite territorial? It’s important to give them plenty of swimming space and choose their tank mates carefully.
I’d recommend only ever keeping a maximum of two angelfish in a 40-gallon aquarium, either alone or with some hardy tank mates. For a school of angels, a 75-gallon tank would be the minimum size.
Let me explain.
First of All, How Big Do Angelfish Get?
The first piece of information we need to know when looking at the stocking density of a type of fish is the eventual size of the species.
Freshwater angelfish for aquariums mostly hail from two different species of medium-large cichlids. The common angelfish (Pterophyllum scalare) has a body length of up to 6 inches, whereas the altum angelfish (Pterophyllum altum) can grow up to 7 inches long.
But adult angels are very unusual in that their bodies are taller than they are long – the slightly larger Altum angelfishes are only 7 inches long but can grow up to 12 inches tall!
For our calculations, we will use this maximum dimension of 12 inches to work out the maximum stocking density.
The Different Dimensions of a 40-gallon Aquarium
Now, it’s good to understand that 40-gallon aquariums come in different dimensions. The three main ones are:
- 40-gallon Breeder Tank : L36″ x W18″ x H16″
- 40-gallon Long Tank: L48″ x W12″ x H16″
- 40-gallon High / Tall Tank: L36″ x W13″ x H20″
So, why the different dimensions? Well, a breeder tank is wide but fairly shallow to allow you easier access to your fish.
People desire long tanks mostly from an aesthetic standpoint. But the extra length sacrifices much of the width, leaving only 12 inches of space for your fish to turn around in.
High tanks look good with tall fish and are useful for those wanting to make a deep substrate for planted tanks. On the other hand, they have less surface area for the fish to move around in.
Which Type of 40-gallon Tank Is Best for Angelfish?
Of the three types of 40-gallon tanks, I’d advise that the tall version is the best suited to angelfish. Since angelfish can reach 12 inches high, a 16-inch depth seems very little to give them enough wiggle room.
Offering them 20 inches of depth allows more swimming space in the vertical column, and may also look more aesthetically pleasing for hosting such a tall fish.
The Dick Mill’s Method for Calculating Stocking Density
Although for smaller fish we can use the one inch per gallon rule, this doesn’t apply to larger species.
Since adult angelfish are medium-large fish, we’ll need instead to use Dick Mill’s Golden Rule of 12 square inches of aquarium floor surface area per inch of fish.
A 40-gallon breeder tank has a surface area of L36 x W13 = 468 square inches.
Now we need to divide that figure by 12 to see how many combined inches of fish we can fit into that space. 648 / 12 = 39.
How many 12-inch angelfish make a combined total of 39 inches? 39/12 = 3.25
Why Three Angelfish Is Too Many for a 40-Gallon Tank
Although our calculation tells us that we can keep three angelfish in a tall 40-gallon tank, I wouldn’t advise it.
Because angelfish like to pair off and breed, keeping three would likely leave you with a breeding pair and one lone fish.
This isn’t good news, because when angelfish decide to breed, they can suddenly become very aggressive fish! The third fish would likely be seen as a threat and attacked.
Additionally, if the third fish was another male, it’d likely become incredibly jealous and try to steal the female, leading to endless fighting between the males. All in all, not a good situation!
Why Two Angelfish Is the Best Number for a 40-gallon Tank
In conclusion, then, a single pair of angelfish looks like the best option for a 40-gallon aquarium. A 40-gallon tank simply isn’t big enough to have multiple breeding pairs, and having three fish could lead to social problems and fighting.
If you can identify the sex of the angelfish before you put them in the tank, you’ll know for sure that you can breed from them.
A breeding pair of angels is wonderful to watch, and the possibility of raising the young is an exciting venture for any fish keeper. But there are some potential issues with breeding angelfish, too, especially in a 40-gallon community tank.
The Problems With Keeping Angelfish in Small Community Tanks
By angelfish standards, a 40-gallon tank is very small. These large, territorial fish need plenty of free swimming space and can become aggressive toward other fish when there’s not enough of it.
To make matters worse, angelfish become even more tetchy when they’re breeding. Because they see other fish as a potential threat to their eggs and young, a pair may even try to clean out a fish tank of all other fish before they spawn!
This depends a lot on each angelfish’s temperament. Some angelfish keepers have had little problems with aggression in community aquariums, even at spawning time.
The risk remains high, however, so you’d better be careful when choosing your angelfish tank mates in such a small tank!
Fish To Avoid With Angelfish in a 40-gallon Tank
Because angelfish can become so aggressive in small aquariums, I wouldn’t recommend keeping any very small fish with them.
Neon tetra, guppies, betta fish, and other similar-sized fish are all best avoided since they would all be too vulnerable to the menacing attacks of brooding angelfish!
It’s also important to avoid keeping other fish that tend to nip fins alongside angelfish. Barbs and certain kinds of tetra (especially serpae tetra) are notorious for their fin-nipping habits!
While the much larger angelfish will always have the upper hand, these smaller fish are more agile and can nip at fins when the angelfish is caught off guard.
Good Tank Mates for Angelfish in a 40-gallon Tank
In general, I’d advise that a 40-gallon tank is fairly small for keeping open water schooling fish with angelfish, and a larger tank like a 55-gallon tank would be much better.
But if you really want to try schooling fish with angels in a 40-gallon aquarium, you’ll need to make sure that they’re fast, energetic, and robust! Pearl danios and giant danios are two possible candidates.
Bottom-dwelling fish species such as corydoras catfish, plecos, and kuhli loaches should also be able to stay out of harm’s way providing there are enough rocky caves and plants to provide hiding places for them!
Keeping an Angelfish Alone in a Community Tank
Angelfish are social fish that enjoy the presence of each other. Ideally, then, they should be kept in pairs or schools.
But because they can become aggressive towards their tank mates at breeding time, some people avoid the problem by keeping a single angelfish in a community tank.
There’s some debate about whether this is a humane treatment for an intelligent, social fish. My conclusion is that as long as angelfish have some suitable tank mates to keep them stimulated, you could keep one solo, but it’s better to keep them in groups.
You can read my specially dedicated article on the topic of keeping angelfish alone here.
Can You Breed Angelfish in a 40-gallon Tank?
If you want to simply keep a pair of angelfish for breeding without any other fish, then I’d recommend 40 gallons as the minimum tank size for a breeding tank. Although some people do breed angels in 20-gallon tanks, this should only be a very temporary situation.
Breeding angelfish is a fascinating but complex endeavor that needs thorough understanding for high chances of success. Because angelfish are prone to eat their eggs and their young, it’s usually necessary to remove the parents from the tank after spawning.
The Best Sized Tank for Angelfish
For keeping a school of angelfish, I’d recommend a 75-gallon tank as the minimum size. In a 75-gallon tank, you can keep six angelfish, which is the ideal number in many ways. I’ve written an article explaining how to keep angelfish in a 75-gallon aquarium here.
Best Tank Setup for Angelfish
To find out all about the best way to set up your fish tank for angel fish, I’d recommend heading over to our angelfish care guide here.
Amazon sword, Crypts, and Java fern are just a few of the tough plants that you can incorporate to help your angelfish feel at home. It’ll also help them to live long, healthy lives, and less stress will also mean less aggressive behavior towards each other, as well as towards other types of fish.
Angelfish are moderately territorial fish, and need plenty of space, especially during spawning time. A 40-gallon is only big enough for a single pair of angelfish, whether alone, or alongside a few robust, but peaceful fish species.
In general, bigger tanks are better for angelfish, and if you want to keep a school, a 75-gallon tank is the minimum size.