How Many Fish Can You Keep In A Reef Tank?


This is one of the most popular questions for a beginner in this hobby. We all want as many fish as we can put in our aquariums, but we know that if we have too many they will begin to die – So what’s the formula?

There is a general rule of thumb, however, there are also many factors that need to be considered that may allow you to keep more or may prevent you from having the fish you would like.

1″ of Fully Grown Fish for Every 5 Gallons of Water Volume

A good Rule-Of-Thumb for beginners with new reef tanks are:

Aquarium Size Total Inches of Fish
10 Gallons2″
15 Gallons3″
20 Gallons4″
30 Gallons6″
55 Gallons11″
75 Gallons15″
120 Gallons24″
150 Gallons30″

This total inches rule of thumb can be split across many fish or you could have just one fish, but you have to consider the environment the fish will live in rather than how many you can put in there.

For Example:
Would you be more comfortable being on a train when every person has a seat, the airconditioning is nice and it’s a pleasant ride home.
OR
Would you be more comfortable on the same train when its standing room only, the air conditioning has broken, its the end of the workday and its 100°F?

There are many factors to consider about the habitat which will dictate your stocking level…

Factors That Alter This Formula

Aquarium Age

The maturity of your aquarium is going to play a huge factor in how many fish it can support. When your aquarium is new there are a few things to be aware of:

  • It has not grown the Nitrifying Bacteria to enough numbers to process the waste
  • It will go through algae blooms that will increase the waste load and consume oxygen
  • You as a new owner may not have the experience to keep your water parameters stable
  • You may not have had enough time to learn how your tank naturally operates and where its parameters settle
  • Your maintenance routine may not be regular enough yet
Richards Reef Dry Rock
A Very New Aquarium – Not Adequate For A High Bio-Load

Just like any job or situation in our lives, we get better the more we do it and the longer we do it. This is called experience. Your aquarium is no different and as your experience grows, your aquarium matures, and you understand it better you will be able to keep more livestock in it.

Bio-Load

Bio-Load or Biological Load is the amount of waste created by every living organism in your aquarium. This waste has to be removed, treated or converted by your 3 filtration types in your aquarium.

  1. Mechanical filtration – This physically removes large particulate waste from the water
  2. Chemical Filtration – This chemically absorbs or converts pollutants, elements or odors within the water
  3. Biological Filtration – This is bacteria that consume and convert the toxic elements in the water into safer elements

The more waste put into the water, the more the filtration has to process. Most of the waste is a culmination of uneaten food, dead and decaying matter and animal excrement.

When your biological filtration is new it has not had time to grow enough bacteria to process the waste. It needs time to grow every time you add a new fish to your aquarium – Hence why you never add lots of fish in one go.

Your aquarium needs to reach a balance of bio-load and bacteria to process it. When your bio-load overwhelms your bacteria this is when you get high Ammonia, Nitrite & Nitrates.

Water Temperature

Although you may not realize this but water temperature plays a huge role in how much oxygen it can store. The warmer the temperature, the less oxygen it can hold.

Oxygen can be depleted rapidly if you have a high bio-load and then your water temperature is allowed to rise, especially in summer.

The dissolved oxygen capability of water at 78°F is 8.4mg/L
The dissolved oxygen capability of water at 82°F is 8.0mg/L

This may not seem very much but if your bio-load is already high and your Dissolved Oxygen is just enough to support your current inhabitants, any rise in temperature could begin to affect your fish.

Water Flow

High, random and chaotic water flow is paramount in a saltwater aquarium, even more so when you begin to add corals. Water flow is what keeps the detritus and waste suspended in the water and prevents it from settling on corals, in holes in the rock, on your sandbed etc.

Gyre Wavemaker Flow Patterns

When the waste is suspended in the water column it is able to pass through to the filtration to be processed. If it is left to settle in the aquarium it can do several things:

  1. Detritus settling on coral can burn the corals’ tissue
  2. Detritus left in holes or on the sandbed will begin to decay and break down. This will raise your Ammonia, Nitrate & Phosphate

Water flow in a reef tank needs to be at least 20x the total water volume.

What this means is the water moving around IN your aquarium needs to be:

___Total Flow Rate of ALL Wavemakers & Powerheads___
Total Volume of Aquarium (& Sump, Refugium if fitted)

eg:
Total Water Volume = 60 Gallons
2 Wave Makers = 750 Gallons Per Hour each = 1500gph

1500 / 60 = 25x Water Flow Turnover – This is an adequate starting point for flow.

You may have noticed I have not included the return pump in this equation. That is because the flow through your filtration and/or sump needs to be alot slower. The flowrate through your sump needs to match the flowrate of your Protein Skimmer to allow it to work most efficiently.

To increase flow rate turnover in your aquarium you add powerheads and wavemakers, not increase the size of the return pump!

High water flow gets the waste to the filtration,
A slower flow rate through the filtration allows the waste to be processed.

For Example:

You want the Check-In line at the airport to move as fast as possible, but you want the check-in agent to be methodical and accurate checking you in so you don’t get your luggage lost or you get given the wrong boarding passes 😉

Adequate Filtration

Adequate of anything is important in life and your filtration is no different. Having enough Nitrifying Bacteria to constantly process waste is the point every aquarium needs to at to maintain equilibrium.

However, you can only fit so much bacteria in a given space. Bacteria live on surfaces in your aquarium, once all those surfaces are covered you can no longer grow any more bacteria. If you now add more fish, you need more bacteria, but there is nowhere for it to grow.

There are ways in which you can provide more space and they will be covered later in this article.

The main focus of this section is you need to be aware that there is a point where you cannot physically get any more cleaning power out of your aquarium.

You will notice when you have reached this point if, with regular maintenance, water changes and testing your Ammonia, Nitrate, and Phosphate continue to rise. At this point, you are overstocked with fish or you need to increase your filtration media.

Swimming Space

Researching your intended fish purchases is something that I cannot stress enough! Spending 10 minutes looking into the requirements, and compatibility of your new fish is paramount to its health.

Swimming space can be an important factor in the fish you have or want. Many fish are natural long-distance swimmers in the wild and require open water to be able to flex their muscles.

This is one of the reasons why Tangs, especially Hippo Tangs become stressed in small aquariums. Tangs, Butterflies, Angels, Triggers, and Anthias all require open space to inhabit the water column where they would naturally. Remember you are trying to create a natural part of the world’s coral reefs in your home.

Designing an open aquascape can be done, but you need to have a plan of the type of fish you want to keep before you even set your tank up. I knew I wanted a Yellow Tang in my 75 gallon reef so I created an aquascape to allow him to swim in the strong current.

My Reef Aquarium
My Reef Aquarium

Allowing a fish to become stressed is a surefire way to get an illness like Ich to show its head and spread to every fish in your tank! If you have a small tank, you need small fish that sit more than swim!

Hiding Space

Every fish you add to your tank needs a home. When a fish is startled or threatened you can see it will bolt to a secure spot. Every fish needs to have a safe spot.

The way you craft your Live Rock aquascape needs to provide hidey holes for every sized fish. Small tanks with small fish are easy, but as your aquarium gets bigger, so do your fish. Again, a plan of what fish you want to add will help create the perfect environment for them.

Types Of Fish

As a beginner, there are many fish that you cannot have, especially if you have a small aquarium. Many fish require very specific feeding or habitat needs, and many will just grow too large for your aquarium.

The best way to find great fish for a beginner is to have a look at the two articles listed in the ‘Further Reading’ section at the end of this article.

The second way to check if a fish is right for you is to look at an online fish supplier like Saltwaterfish.com. Each fish listing shows the requirements you need to meet as well as the minimum tank size to hold each fish.

Fish Compatibility

When enclosed in a small environment it is important to ensure all your inhabitants will peacefully co-exist. Many fish species can be very territorial and if allowed will make the entire aquarium their territory. Any fish added after will be chased and possibly killed.

To ensure harmony I highly advise you to check your intended fish purchases with the fish you already own to ensure no compatibility problems.

Saltwater Fish Compatibility Chart

Ways To Increase Fish Quantity

Now you have got more information on other factors effecting how many fish you can have here are a few things you can do to help increase your fish quantity over time.

Remember these are all guidelines and testing your water parameters will be the only definitive way to see how your tank is doing with the bio-load.

Time

Patience is the true foundation in this hobby and the longer you hold off adding fish, the better your aquarium’s overall health will be. By learning to walk before you can run you will find that in time you will be able to add another fish, or pair or shoal (Depending on the size of tank).

Let your biological filter grow, your routine refine and your parameters stabilize then you will have a far better success rate than trying to get all your aquarium up and running and stocked to the brim in the first 3 months.

Sump

Adding a sump is a great way to increase water volume and provide space for adding more live rock to increase the surface area on which Nitrifying Bacteria can colonize.

Increasing water volume will also help to smooth out any fluctuations in water parameters as there is more water volume for that influence to work on.

MarinePure Blocks

If you have room in your aquarium filter compartment or in your sump I highly recommend adding some MarinePure. These come in a range of plates, blocks, spheres and small gems and they are products to provide immense surface area for your Nitrifying Bacteria to colonize.

They are a super porous material that have an insane square footage of surface area due to the pores, holes, tunnels, and tubes throughout each piece.

More Nitrifying Bacteria = Higher Bio-Load

Maintenance

Regular maintenance is one of the best ways to ensure the maximum bio-load for your aquarium can be reached.

Weekly vacuuming of the sand bed, blasting settled detritus from your rocks with a turkey baster, regular changing of the filter socks/floss and timely changing of chemical filter media will ensure your aquarium remains clean and healthy.

If your aquarium is healthy, your livestock will remain healthy.

Water Changes

The quickest and fastest way to keep Ammonia and Nitrate low is weekly water changes with high-quality RO/DI water and a good salt mix from a reputable brand.

If your water changes are only once a month then that allows time for these toxins to build and stress your fish. With a regular supply of new saltwater, your parameters will remain stable and your water quality will be higher.

Higher Water Quality = Higher Bio-load

Smaller Feeding

Many aquarists think they need to feed more than they should, your fish are pigs and they will eat all the time if they could, but large or lots of feedings can really overload your biological filter.

Not all food gets eaten and if it is allowed to settle it will break down and raise your water parameters. To also add to the problem is the more you feed your fish, the more they go to the bathroom.

Fish waste allowed to settle will also break down and raise your water parameters.

This where I find blasting your rocks with a turkey baster can really help get this junk into the water column to be removed by your filtration and water changes.

Feeding small amounts twice a day and watching to ensure it ALL gets eaten is better than dumping in a load ful once a day.

Clean-Up-Crew (CUC)

The final part of a healthy aquarium – The janitors. Shrimp, crabs, snails, and worms are all scavengers and they really do a great job of helping to keep detritus and fish waste to a minimum.

Cleaner Shrimp
Cleaner Shrimp

By constantly scouring the aquarium for food they keep your tank clean and help to offload some of the work of your biological filtration.

Conclusion

The maximum amount of fish you can have in your aquarium will all depend on many of the factors listed above. Waste management, ecological balance, fish habitat, stress, health, compatibility & maintenance all play their part in maximizing the amount of fish you can stock.

With patience and some careful fish selection, you will have a beautiful reef tank with enough fish to create a wonderful addition to your home or office.

By starting out with 1″ of fish for every 5 gallons you will be on track for success.

Further Reading

If you found this article helpful please be sure to check out some of our others:

Richard

Hi, I'm Richard and I have been an avid aquarist for over 30 years with a passion for Saltwater Aquariums. I love to pass on my knowledge to help others get the same amount a pleasure out of this hobby as I do. View my About Me page to find out more about me & my mixed reef aquarium.

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