There are hundreds of different saltwater fish that are suitable for life in an aquarium and one aquarium can contain an incredible variety of species. The exact number of fish you can keep in a particular tank is governed by a few important factors including the tank size, available habitat, size of the fish, and tolerance of tankmates.  When stocking a new aquarium, it is very important to consider ALL of these factors if you want to ultimately achieve the best variety of compatible saltwater fish. 

Tank Size

An aquarium is only so big and can only support so much biomass. Whether you are keeping x10 small reef fish that weigh 10 grams each or x1 larger fish that weighs 100 grams, you can only support 100 grams of total fish. The larger the water volume, the more biomass you can keep but there are some nuances to consider.

The variety of fish you can choose from may be limited by the tank's dimensions. Longer tanks provide more usable space and present the largest variety of different fish to choose from. Smaller tanks with limited swim room will eliminate large fish that need plenty of space to feel comfortable.  

For example, adult tangs grow quite large and should be kept in a tank measuring at least 48"-72" long. While you might be able to keep x10+ smaller reef fish in a 30-gallon tank that only measures 24" long, a single adult tang won't survive because the dimensions of that tank are not conducive to the fish. That means tangs are off the table if you own a small 30-gallon tank, limiting the pool of suitable fish. 

Fish Size

You should always consider the ADULT size of the fish. Juvenile fish will grow and you want to be sure you have the space to house the fish throughout their entire life. Since the aquarium is only so big, you can either keep multiple small fish or just a handful of large fish.  Often times it is a mixture of different size fish that will allow for the best variety.  


This is how well the fish tolerates tankmates. A predatory fish that eats other small fish will obviously limit the types and size of fish you can keep alongside it. Generally speaking, smaller bite-size reef fish and small invertebrates should not be housed with larger predatory fish because they might get eaten. 

Some fish prefer to be in small species groups or pairs while others may prefer to be solo. Many fish species are territorial and will only show aggression when other fish intrude on their space, this is especially common with fish of the same or similar species. This leads us to the next limiting factor which would be available habitat.  

Available Habitat  

Different fish will inhabit different areas of the tank. Some like the sand, while others like the rocks or swimming in open water up above the rocks. A fish will often protect their little niche in the tank as their own or be quite stressed if they don't have that preferred space due to competition. Spreading out your fish among the different habitats will allow you to keep a greater variety as opposed to overcrowding one particular niche in the tank. 

Learn More with BRStv: How To Choose Beginner Saltwater Fish Wisely

Successfully stocking your tank with the maximum number of fish takes some effort and in some cases, a fish that is a model citizen in tank A will be a complete bully in tank B so the conditions can change from tank to tank. The best approach is to do the appropriate research around the particular fish's requirements. Avoid purchasing fish on impulse and understand that some conflicts may occur no matter how much research you do.  If you find that you have overstocked your aquarium for any reason, upgrading your tank and/or filtration is always an option. Building a 2nd tank is also a great opportunity to keep a specific species of fish that may not get along with your existing population.