Ahhh…..the question of all questions! This has to be the most common question we get when consulting for a new aquarium. You may have heard some formulas about inches of fish per gallon and other ratios of some sort, but those are general guidelines at best and could be misleading and inaccurate. In this article, we will dive a little deeper and explore this question in more detail and with common sense. Because our knowledge and experience is mainly with marine environments, we will focus on saltwater aquariums.

Factor 1 – Water Quality

The first and most important factor to consider is how water quality will be affected by the number of fish in your tank. Every aquarium has it’s limit when it comes to the biological load. Regardless of your aquarium size and what may be recommended, I would focus on having as many surfaces as possible for aerobic bacteria to live where it can consistently handle ammonia and nitrite conversion into nitrates. When I say surfaces, I refer mostly to filtration media such as the traditional plastic bio-balls, ceramic media, live rock, dry rock, etc..the more you have the better your aquarium will process waste. For those of us fortunate enough to have plenty of space below our tank or in other adjacent spaces, I would also strongly recommend media in low flow areas where more biological media could be placed to encourage anaerobic bacteria to grow – that will give you the added benefit or nitrate conversion to nitrogen which is harmless in the aquarium and dissipates on it’s own. By doing the above, you could make sure that your aquarium has what it takes to handle your water volume many times over. For example, if I were to set up a 200g saltwater aquarium, I may have enough biological media in the sump to handle a 1000g – why not? The expense is reasonable and the benefits are great!

Factor 2 – Fish Size

This is just common sense…bigger fish eat more food and in turn take bigger shits (forgive my french). Unless you want skinny sick fish, you will need to feed your fish a healthy diet that is of the right proportion and frequency. A purple tang the size of a dinner plate creates a lot more waste than one the size of a fist. So…plan ahead and know how big the fish can get – or use it as an excuse to get a bigger tank later :) And, size matters, one could argue that you can place 5 small blue tangs in a 75g tank and at the same time the same size tank would be too small for a large adult blue tang.

Factor 3 – Fish Species

Some species create more waste than others. For example, Tangs are grazers. They want to graze all day and poop all day. Other species like Anthias need to eat more frequently than other fish. Some fish are just “normal” eaters where they will eat the food you give them once a day and be happy and create less waste such as clownfish and other smaller species like gobies and blennies. What about predatory fish like sharks, rays, triggerfish, puffers, etc…? These larger fish have a voracious appetite and certainly create more waste than other species of the same large size. So..know the species you are going to keep and consider their eating requirements.

Factor 4 – Aggression and Territory

Another factor to consider is the aggression of the fish. Some fish may be so aggressive or predatory that they will not allow other fish to coexist with them in the tank. Ever place a small lionfish in a tank only to have him get huge and eat all the smaller fish? Or hog all the food? Ever tried to add a yellow tang to an aquarium that has a powder blue tang in it already? These things matter greatly, so know the species and how it will behave towards other fish in the tank. Sometimes, and this may appear counterintuitive, more is less :) Sometimes the chaos of more fish divides the aggression down among many fish instead of towards one fish. I remember staying away from Powder Blue tangs for years because I saw one in my own tank harass and kill a yellow tang. But now, after gaining some experience, we use them all the time and make sure we have 3-5 yellow tangs – not just one. The same goes for Anthias, you want one male for 5+ females so he doesn’t harass a single female to death. Territory is also important yet often overlooked. Make sure you have plenty of hiding spaces and territories for fish to claim. Some aquarium enthusiasts prefer an open aquascape with very little in the tank which may work fine, just make sure you have the right fish species where territorial disputes won’t be an issue. You want to avoid several fish all fighting to hide in a single stack of rocks.

Scenario 1 – 20 gallon Nano Aquarium

Let’s look at a very common scenario for beginners… a 20g nano tank. I would recommend a pair of ocellaris clownfish, a small hardy blenny species, a fire goby would be great as well. You might be able to get away with a couple of more species if you are on top of the maintenance often. Stick with non-aggressive smaller species and stay away from fish that require more swimming space like tangs.

Scenario 2 – 90 gallon aquarium

A very common size for sure and one that is common in apartments, the standard size is approx 48″ x 18″ x 24″. Here is where species selection is relevant. You could easily keep 10-20 small species of fish in this size tank – nonaggressive and less demanding species like clowns, gobies, blennies, pygmy angelfish, etc. Alternatively, you may decide to do more aggressive predatory fish like an eel, a small puffer, trigger, etc.. In this scenario, you probably could only get away with 3-5 fish and even then, there’s a good chance they will outgrow the tank in a couple of years. So what do you want? 20 fish? 5 fish? This is a perfect example where the number is meaningless – know the species and know the limitations of your aquarium and stock it accordingly.

Scenario 3 – 180g gallon aquarium

This larger size aquarium is usually an upgrade from a smaller one :) I know it was for me…it’s a great size that can fit most homes and is 72″ x 24″ x 24″. Looking at all the factors we discussed above and starting with water quality…this larger footprint of a tank will allow you to have more than enough media in the sump to easily handle a large bio load. Just placing 4 ceramic blocks in the sump will be enough for a 2000g tank! When it comes to fish size, most available species will work here and do well with the exceptions of some very large predatory fish. As far as species, you could do a large number of community fish of all sizes – imagine a school of 15 damsels, 15 anthias, tangs, angels, butterflyfish, and other medium to large fish – you could end up with 50 fish – all doable as long as you have good balance of small and medium\large fish. Otherwise, you could choose to do larger species combined with predatory fish, so imagine an eel, a few triggers, large angelfish, large tangs, rabbitfish, etc..given these larger and more demanding fish you may end up around 10-20 fish. Either one would look great – it’s all about what fish you want to keep. Lastly, consider the order of the fish additions and their aggression level – add all at once or gradually with the most aggressive fish last. Make sure you have plenty of territories for the fish as well.


So..there you have it. You may have not gotten a straight short answer that many of us seek, but I hope the guidelines and examples listed above help you make the right decision and lead to a healthy thriving tank. Remember to research the species you want to keep and talk to others that have kept them before – observe your aquarium and the fish behavior and you will have the common sense to know if you have maxed out the fish capacity in your tank.