How many fish should you have in a saltwater tank?

When setting up a new saltwater aquarium, one of the most common questions is “How many fish can I put in my tank?”. While there is no single definitive answer, there are some general guidelines to follow when stocking a marine aquarium with fish.

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What factors determine how many fish your tank can handle?

The number of fish that can be successfully kept in a saltwater aquarium depends on several key factors:

Tank size – The total volume of water in gallons. Bigger tanks can safely hold more fish.

Filtration capacity – More powerful filters with higher flow rates can process more waste.
Types of fish – Small, peaceful fish have lower bioloads than large, active fish.
Live rock and decor – More rock and decor provides more usable surface area for beneficial bacteria.

Performing regular maintenance – Weekly water changes and filter cleaning lowers nitrate buildup.
Avoiding overfeeding – Feeding less reduces waste production.

By balancing all these elements, you can optimize the number of fish in your saltwater tank.

Recommended stocking levels based on tank size

As a very general guideline, the following table provides suggested starting points for how many fish can be maintained per tank volume:

Tank Size	Light Stocking Level	Moderate Stocking Level	Heavily Stocked
10 gallons	1-2 small fish	2-3 small fish	3-5 small fish
20 gallons	3-5 small fish	5-7 small fish	7-10 small fish
30 gallons	5-7 small fish	8-10 small fish	12-15 small fish
40 gallon breeder	8-10 small fish	12-15 small fish	15-20 small fish
55 gallons	10-12 small fish	15-20 small fish	20-25 small fish
75 gallons	12-15 small fish	20-25 small fish	25-30 small fish
90 gallons	15-20 small fish	25-30 small fish	30-40 small fish
125 gallons	20-25 small fish	30-40 small fish	40-50 small fish
180 gallons	25-35 small fish	40-50 small fish	50-60 small fish

The categories represent lightly stocked (beginner level), moderately stocked (intermediate level) and heavily stocked (advanced level).

The term “small fish” refers to species that reach 3 inches or less at adulthood. For larger fish, the number of fish suggested for each tank size would need to be reduced accordingly.

General guidelines for stocking saltwater fish

In addition to tank size, it’s important to follow these general rules of thumb when adding fish to your saltwater aquarium:

Add fish slowly over time. Never add more than 3-4 small fish at once in a new tank.
Target 1 inch of adult fish length per 2-5 gallons of water.

Allow 1-2 weeks between adding new batches of fish to let the biological filtration catch up.
Avoid overstocking by following the inch-per-gallon rule. Overcrowding causes stress, aggression and poor water quality.
Select fish that are compatible in size, temperament and needs. Don’t mix aggressive species with docile ones.

Research the adult sizes, behaviors and care needs of fish before purchasing.
Include clean-up crew animals like snails, hermit crabs and shrimp to help eat uneaten food and algae.
Quarantine new fish for 2-4 weeks to avoid introducing diseases.

Tips for stocking community fish tanks

Many hobbyists prefer to set up community tanks containing multiple species that coexist peacefully. Here are some tips for stocking mixed reef community aquariums:

Choose most fish from tight-schooling species that exist in large groups in the wild, such as damselfish, chromis, wrasses and anthias.
Include bottom dwellers like blennies and gobies.

Add some larger semi-aggressive fish like triggers and angels to serve as centerpiece specimens, limiting to 1 per tank.
Include herbivores like tangs and rabbitfish to graze on algae.
Allow 1-2 inches of adult fish per 10-20 gallons for a community tank.

Ensure any semi-aggressive fish have plenty of rockwork to establish territories and reduce conflict.

Stocking options by fish family

Here is an overview of how many fish from popular marine families are reasonable to keep in tanks of different sizes:

Clownfish

10-20 gallon tank – 1 clownfish pair

30+ gallon tank – 2 clownfish pairs

Damselfish

10-20 gallon tank – 1-2 small species (i.e neon velvet damsel)
30-40 gallon tank – 3-5 small species (i.e 3-stripe damsel)

55+ gallon tank – 6-8 small species

Dottybacks

10-20 gallon tank – 1 dottyback
40 gallon tank – A pair of dottybacks OR 2-3 of smaller species like orchid dottybacks

75+ gallon tank – 2-4 dottybacks depending on species and aggression level

Blennies

10-20 gallon tank – 1 blenny
40 gallon tank – 2 smaller blennies or 1 larger species (i.e lawnmower blenny)

75+ gallon tank – 3-5 blennies of assorted species

Assessor/Cleaner Blennies

10-20 gallon tank – 1 small shrimp goby paired with 1 assessor/cleaner shrimp
40 gallon tank – 1 assesor/cleaner shrimp with 1-2 shrimp gobies OR 1 assessor/cleaner blenny

75+ gallon tank – 1 assessor/cleaner blenny AND 1-2 shrimp gobies with paired shrimp

Gobies

10-20 gallon tank – 1-2 small shrimp gobies paired with shrimp
40 gallon tank – 2-3 small shrimp gobies with shrimp OR 1 larger goby (i.e. diamond watchman)

75+ gallon tank – 4-6 small shrimp gobies and 1 larger goby species

Tangs

10-20 gallon tank – None
40 gallon tank – 1 small species (i.e. bristletooth tang)

75+ gallon tank – 1 medium tang (i.e. sailfin/deshazor’s tang)
125+ gallon tank – 1 large tang (i.e. blue hippo)

Angelfish

10-20 gallon tank – None

75+ gallon tank – 1 dwarf/flame angel
125+ gallon tank – 1 medium angelfish (i.e. lemonpeel)
180+ gallon tank – 1 large angelfish (i.e. bluegirdled)

Butterflyfish

10-40 gallon tank – None
75+ gallon tank – 1 small species (i.e. sunburst butterfly)
125+ gallon tank – 1 medium species (i.e. raccoon butterfly)

180+ gallon tank – 1 large species (i.e. saddleback butterfly)

Triggers & Filefish

10-40 gallon tank – None
75+ gallon tank – 1 small species (i.e. pinktail trigger)

125+ gallon tank – 1 medium species (i.e. clown trigger)
180+ gallon tank – 1 large species (i.e. blue throat trigger)

Tanks Over 200 Gallons

Very large tanks allow for more fish but require advanced care.

Limit tangs to 1 per 100-125 gallons.
One medium-large angelfish or butterflyfish per 125 gallons.
No more than 1 triggerfish.

Can mix in larger aggressive fish like puffers, eels, lionfish, groupers.
Increase numbers of smaller schooling fish like anthias and chromis.

Other livestock to include

In addition to fish, saltwater tanks should also be stocked with important cleaner organisms:

Clean-up snails like astrea, margarita and turbo snails help eat nuisance algae and detritus. Allow 1 snail per 5 gallons.
Hermit crabs scavenge uneaten food and debris in the substrate. Stock 1 crab per 10 gallons.
Cleaner shrimp pick parasites off fish and help break down waste. Add 1 per 20-40 gallons.

Crabs/shrimp add interest and fulfill clean-up roles. Allow 1-2 small species per 20-40 gallons.
Sea stars are great scavengers but some species may eat desirable invertebrates. Use caution and research compatiblity.
Brittle stars are excellent detritivores but may proliferate. Limit to 1 per 40 gallons.

Urchins graze algae but have specific needs. Only add compatible species like pencil urchins.

These animals provide important janitorial services and should be accounted for in the tank’s overall bioload.

Water parameters

To support a higher fish population, closely monitor water parameters and utilize protein skimming, ozone and frequent water changes to prevent problematic build-up of metabolic wastes. Target levels should be:

pH – 8.1 to 8.4
Alkalinity – 7-12 dKH (120-200 ppm)
Calcium – 400-480 ppm

Magnesium – around 1300 ppm
Nitrate – Less than 20 ppm
Phosphate – Less than .03 ppm

Ammonia/Nitrite – Undetectable

Perform 10-15% weekly water changes. Test alkalinity, calcium and nitrate weekly.

Conclusion

Determining ideal fish stocking numbers is part art and part science. While the inch-per-gallon rule and guidelines based on tank size provide a starting point, successfully maintaining any aquarium long-term requires a balanced ecosystem with adequate biological filtration. Providing proper environmental conditions and adhering to the specific needs of your livestock will allow you to safely house these amazing reef organisms.