How Many Fish Can I Have in a – – – Gallon Tank?

We’ve all asked this question at some point, and it gets asked a lot on the forums by hobbyists of all levels. How many fish can we have in our tank? Maybe we’ve kept a successful nano tank with 4-5 fish for a few years, and as we plan a larger system we ask, “how many fish can I keep in a new 250 gallon tank?”. That’s a little like asking “Im going on a road trip, how many kids can I fit in the back of my mini-van?”. It depends. How is the back of the mini-van setup? Is it open space or is there seating? How big are the kids? Do they get along? Or are they going to fight with each other? How many restroom stops do you want to make? Do you want a quiet, peaceful drive on this road trip? Or a nightmare, stress-fest of a drive? More kids equals more work. But surprisingly we all ask, how many more fish can I have!? Well the same is true of our tanks. More fish equals more work.

More fish = More Work

Now this is somewhat relative. Adding two fish to a 400 gallon tank isn’t going to have the same impact as adding those two fish to a 10 gallon tank. But ultimately, adding more fish increases the tank’s bioload, and it will eventually reach a point where more is required from us to keep things within acceptable parameters. This could mean more and larger water changes, adding nitrate or phosphate removal media reactors, changing filter socks and emptying the skimmer cup more regularly and so on. So how do we answer this question when planning a new system?

This easy to maintain 25 gallon tank has 3 fish and a small amount of rock work.

The Importance of Planning Ahead

Again, it depends. How will the tank be setup? With lots of rock? or with lots of open swimming space for schooling or larger fish? How big will the sump be? Will that significantly add to the total water volume of the system? How big of a water change can you do? How regularly do you want to do water changes? Weekly?  Bi-weekly? Monthly? How big are the fish you plan to add? Are you thinking of one large hippo tang or eleven anthias? There are so many factors to consider, it really depends on you on what you’re planning. A 100 gallon display tank with a 30 gallon sump underneath it, is not the same as a 100 gallon display tank with a 400 gallon sump setup in the garage or a fish room behind it.

Tightly packed reef tanks with lots of live rock leave little room for active fish like anthias and wrasse.

Be Reasonable, Plan Carefully

Try to avoid rules that offer a number of gallons per inch of fish. These ‘rules’ can be a helpful starting point in some circumstances, but 4″ of one hippo tang are very different from 4 x 1″ chromis when you’re stocking a 20 gallon tank. Tangs are notoriously greedy and messy eaters, creating a lot of waste. However, a goby of the same length is a relatively sedentary fish and will likely have less of an impact on your tanks nitrates, phosphates and nutrient levels. Rather than look for rules, try to think reasonably about what the fish you’re going to keep require to thrive and aim to stay under what you think the ‘max’ is for your tank.


Hippo Tangs can grow very large and need a lot of swimming space.

Careful planning and forethought is needed. Are any of the fish on your list aggressive, territorial or predatory? How will that number of fish affect your water change schedule? and so on. Once you have a plan in mind, add your livestock slowly, allowing time for each new addition to settle in and see how it affects your nitrates and phosphates over a few weeks or perhaps months. Each system is a little different and you want to ensure that you don’t overstock or overcrowd your tank. Do your best to use good judgement when it comes to stocking your tank. Successful reef keepers know the advantages of under stocking their systems, and their mini-vans. Happy reefing!