This question comes up a lot among people who are new to the hobby. They want to create an aquascape that’s absolutely bursting with vibrant fish, but they have a hard time evaluating how much fish can safely coexist in a single tank. Unfortunately, there’s no simple equation that you can apply to every situation. Each species of fish is different, so whatever rules apply to one fish probably won’t apply to another fish. The good news is that there are a few basic guidelines that can help you make the best decision for your aquascape.
One Inch per Gallon
You’ve probably heard this rule a few times. It states that you should have no more than one inch of fish for every gallon of water. That’s not a bad place to start, but this is more of a general guideline than a rigid rule. Why doesn’t it work? Well, think about it like this: suppose that you were cooking a meal for a bunch of people and you estimated that for every five feet of height among your guests, you needed one pound of food. That’s a fairly good starting point, but the problem is that each person is different. A guy who’s five feet tall and weighs 200 pounds will eat way more food than a petite woman who’s five feet tall and 100 pounds.
The same is true for fish. Even though two different species of fish might both be five inches long, one might require much more space than the other.
Do Your Research
I know that reading pedantic books about fish biology isn’t terribly exciting, but it’s an essential step for cultivating a strong aquatic ecosystem. Different fish have different space demands for different reasons. For example, tangs are fairly active fish that require more elbow room (or fin room, as it were) than static fish like neon tetras. Also, schooling fish might need extra space because they tend to bunch together and swim around in a gigantic mass. Understand the unique demands of all of your fish so that you can plan accordingly.
Think for Tomorrow
When it comes to preparing for the future, it’s always a good idea to be a pessimist. Expect bad news so that you will be prepared for the absolute worst case scenario. You need to remember that fish grow and change over time, so even if five fish get along nicely in your aquarium today, it might feel a little bit crowded a few months from now when all of those fish have grown to twice their size. Whenever you add a fish to your aquarium, you should imagine what the living conditions would be like if every single fish was fully grown and required the most amount of upkeep.