Stocking a Tank

There’s more to stocking an aquarium than just fitting fish into a tank. While space requirements are a big part of the equation, they’re only one piece of the puzzle. Water chemistry, fish behavior, waste load, and territorial needs all play a role when you put multiple fish together in a closed environment.

Overstocking is one of the most common mistakes new aquarists make—sometimes unintentionally. Whether it’s adding too many fish too quickly, or choosing species that just aren’t compatible long-term, poor stocking choices can lead to stress, disease, and even death. Ultimately, there’s no universal formula for what makes a tank “properly stocked.” Every system is unique, and so is every fish. This is why careful planning and a solid understanding of your tank’s limitations are so important.

A key principle to keep in mind is that the needs of the fish should always come before your aesthetic preferences. Just because a fish looks cool doesn’t mean it belongs in your tank. Fish are living, breathing animals—not decorations. If you're not prepared to provide the right environment—including tank size, filtration, and appropriate tankmates—then that fish isn't the right choice for you.

Responsible fishkeeping starts with honest decision-making. If a fish requires more space than you can reasonably provide, it’s okay to admire it from afar. There are countless beautiful, fascinating species that thrive in smaller or simpler setups. Choose fish that fit your tank—not the other way around.

Physical Space

One of the most obvious—yet most overlooked—factors when choosing fish is the amount of physical space you have to work with. Fish swim. That’s what they do, and it’s what they’re built for. They need room to move around, explore, and establish territory. Some species are more active than others and require even more swimming space just to live comfortably.

For tropical freshwater fish, a common rule of thumb is about 5 gallons per inch of adult size. That might sound like a lot, but it accounts for more than just the fish itself. It considers waste production, swimming room, and the tank’s ability to maintain stable water parameters. Most tropical freshwater species come from relatively calm lakes, ponds, and rivers—they’re not built for cramped tanks or strong currents.

Saltwater fish tend to be much more active. These species are constantly on the move, and they generally need tank lengths of 6–12 times their adult body size to thrive. That’s not just about comfort—it’s about their physical and mental health. A fish stuck in a tank that’s too small is like a dog stuck in a crate all day. Even if the water chemistry is perfect, they’ll be stressed.

When in doubt, do your research. Look up the adult size of the fish, their swimming habits, and their temperament. If that information isn’t clear, it’s always better to err on the side of giving them more room than less.

As for how many fish to put in a tank? That comes down to judgment. If the tank is visibly crowded and the fish are constantly bumping into each other or struggling to find their own space, it's time to scale back. There’s no perfect formula—just observation, experience, and a willingness to prioritize the well-being of your fish. Fewer fish in a well-maintained tank will almost always do better than a crowded tank running on the edge.

Behavioral Factors

Some fish are blissfully unaware of their surroundings. They’ll swim around without paying much attention to the other fish in the tank. Others, though, are much more observant—and often territorial. Even fish that are generally considered peaceful can turn aggressive if they're overcrowded or housed with the wrong tankmates.

That’s why understanding the behavior of the species you’re keeping is just as important as knowing their size or water needs. A tank that can comfortably hold a large number of one type of fish might only be able to support a single individual of another species.

Take Bettas, for example. They’re similar in size to Platies. In a 10-gallon tank, you could easily keep six Platies with no issue. But try that with six Bettas—even all females—and you’re asking for trouble. The tank might physically fit them, but behaviorally, it just won’t work.

Even in tanks without outright aggression, fish still like their space. Some species are more solitary by nature and become stressed or defensive if they can’t get away from the crowd. A fish constantly surrounded by others may lash out—not because it’s mean, but because it’s overwhelmed.

This is where aquascaping becomes more than just decoration. Adding plants, rocks, and other structures to break up the line of sight can help fish feel more secure. It’s the same concept as having walls in a house—you wouldn’t want to be able to see the bathroom from the dinner table. Fish are the same way. Even social species need a place to retreat and relax.

Chemical Considerations

For most practical purposes, an aquarium functions as a closed system. Life itself, if you zoom out far enough, is just a complex series of chemical reactions. And in any closed system full of chemical activity, entropy becomes a major consideration.

Entropy is a term from thermodynamics that refers to the inevitable loss of usable energy in a system. Every time energy is converted—say, from chemical energy in food to motion or metabolism—some of that energy is lost, usually as heat. In a truly closed system, entropy keeps building until there’s no usable energy left and the system can’t sustain life.

The good news? Your aquarium isn’t a completely closed system. The only truly closed system is the universe itself. With regular water changes and careful management, you can keep entropy at bay in your tank. But if you have too many fish producing too much waste for the volume of water and filtration to handle, you’ll start seeing the effects of entropy: rising waste levels, falling water quality, and an unstable system.

In most aquariums, nitrates are the primary “end product” of this entropy. Now, nitrates can be used by aquatic plants, and it is possible to build a self-sustaining tank. But most hobbyists want more fish than such a setup could naturally support. And here’s the hard truth: if you're putting food into the tank, it's no longer a closed loop. You have inputs—fish food, livestock, plant fertilizers—but unless you're exporting those inputs (like via water changes), they’ll accumulate over time.

Just because waste changes form doesn’t mean it disappears. That would violate the first law of thermodynamics—matter and energy can't just vanish. So even if you're running a heavily planted tank and don’t see much nitrate buildup, water changes are still necessary, because other compounds are accumulating or being depleted in ways we can't always measure.

This is where nitrate testing comes in. It’s not just about toxicity—although extreme nitrate levels can certainly stress or harm fish. It’s about keeping tabs on the balance of the system. If nitrates are building up faster than they’re being removed, that tells you your system is chemically overstocked.

The general consensus is that nitrates should stay below 80 ppm in a freshwater tank, and ideally much lower. If you’re doing regular maintenance and feeding responsibly but still can’t control your nitrate levels, that’s a clear sign it’s time to reduce your bioload. Different species contribute differently—some are messy eaters or produce more waste, others less so.

Even though nitrates aren’t immediately toxic, they act as a useful barometer for how balanced your tank is and whether you're keeping up with necessary maintenance. In that sense, they’re a warning light for entropy creeping in—and a reminder that the tank needs your help to keep from falling into disorder.