How Fast Should You Stock The Aquarium

Throughout the years lots has been written about the stocking rates in saltwater aquariums – some of this information has been very informative and unfortunately some has been very misleading.

Misleading information provided has been along the lines of ‘1 cm of fish for every 5 litres of water’, ‘1 cm of fish for every 13 litres of water’ etc.

As you can see just from the above two examples it can be very misleading!

On top of the above there has been limited differentiation between fish only aquariums, coral only aquariums and mixed reef aquariums!

The main trouble is when it comes to stocking the aquarium – each aquarium is different. Each aquarium has different equipment, different animals etc. Even if you have two aquariums with exactly the same equipment then they would still be variances.

Overstocking in an aquarium is unfortunately something that the majority of aquarists do. For a long time they get away with it until something changes and the aquarium starts to go downhill. Basically if the correct care and maintenance is performed on a weekly basis and no equipment fails then some aquarists may be able to get away with overstock – until some area of the care and maintenance is missed, a piece of equipment fails and then the trouble starts.

The issue is not just with the aquarium being overstocked. Another issue is when the aquarium is stocked too quickly. This could either be that fish are introduced into the aquarium prior to the aquarium finishing its cycle, too many fish are introduced at the same time or not enough time is left in between the introduction of fish.

The first thing that the aquarist needs to ensure is that the aquarium is ready for the introduction of fish. By this I mean that the filtration in the aquarium is ready to process the waste created by the fish, uneaten food etc. This, as already mentioned is the cycle. In the cycle Ammonia is converted to Nitrite which is in turn converted to Nitrate. Both Ammonia and Nitrite are toxic to fish therefore no readings of these should be evident in the system.

At first testing only for ammonia will suffice, then testing for nitrite. When the ammonia levels reduce the nitrite should increase which should then reduce only for the nitrate to increase.

Nitrate is not toxic to fish but should be controlled either by water changes, natural methods etc to ensure that it stays within the recommended parameters for the type of aquarium you keep.

Once you have no readings at all for ammonia or nitrite then the aquarium is ready for the introduction of a fish – just the one don’t get greedy and overdo it. I appreciate that this is hard as there will be lots of beautiful fish in the shop which you could walk out with but you need to resist the temptation.

Before you go out to the shop though you need to realistically have a think about what fish you are hoping to keep in the aquarium (as well as corals and invertebrates if you are hoping to keep these). When you are thinking about this consider talking to more experienced aquarists or a couple of aquarium shops who you have started to build a bit of trust with. In my opinion it is good to build a good relationship with your local fish shop as they will get to know both yourself and your aquarium and you never know when you may need them.

When thinking about your list what you want to ensure is that all the fish you hope to keep will live happily with each other (and corals/invertebrates if you are keeping them).

There is something else you need to think about at this point – the hardiness of the first fish.

It is recommended that a hardy fish be introduced first into the aquarium at the start of its life. In my opinion this is correct as everything in the aquarium is new and basically the water is raw and too clean. A hardy fish is good but what you do not want is a territorial fish. A good example of this is the damsel fish. Whilst the fish is hardy it will quickly believe that the entire aquarium is its territory and when you introduce other fish into the aquarium they could easily get bullied which in turn leads to stress, disease and possibly death.

What you are looking for is a fish which is hardy yet is not territorial.

A good example of this is the common clownfish – tank bred if possible.

So let’s pretend at this point that you have gone to your local fish shop and ignored all the other fantastic fish and purchased just the one – a common clownfish.

Well done to you!

Now you have it home and you need to follow the correct acclimatisation procedures. A lot of people ask me if they need to quarantine their first fish. In my opinion there is no need to do this as there is nothing else in the aquarium which could be effected if this one becomes sick but if you are on the cautious side then quarantine the fish for a week – it won’t do any harm.

One the fish has been acclimatised to your aquarium it should soon start to explore, take food and perhaps even become bold.

At this point you may become tempted to go out and purchase one, two, three etc more fish. You should not do that though. You need to test the water. Prior to adding the first fish to the aquarium the filtration had nothing to deal with other than the cycle and any areas of die off which may have occurred. Now that a fish is in the aquarium you will be feeding it and not all that will be eaten and will fall to the bottom. What is eaten will eventually pass out of the other end and needs to be dealt with.

All this is processed by the filtration and the filtration needs time for the bacteria levels to increase. This is what you are waiting for – stability in the filtration.

This can again be done by testing for nitrite and nitrate. Even if these readings are zero for nitrite I would still wait a couple of weeks prior to adding a new fish. Give the aquarium time to settle and enjoy your new fish.

Once your readings are ok and at least a couple of weeks have passed then you can go and get another fish if you want to. Basically at this point you do the same again. Test the water and wait for stability.

Basically you should stock the aquarium very slowly. The quicker you stock it the more load you are putting on the filtration system and it may not cope. That in turn will mean that you may get ammonia or nitrite readings which are not good and as said are toxic to fish.

Also don’t forget that the faster you stock the aquarium the quicker it will get full. Enjoy stocking it and take your time.


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  1. I have 4 fish 2 common Clownfish and 2 Black & White Ocellaris Clownfish. they were great fish considering I didn’t know my about cycling a tank when I got them. You made a good point that the fish community has really changed the way they feel about cycling a fish tank with fish. For the most part its considered cruel now to your fish to put them through the cycling process.

    fishs last blog post..Unhappy Fish!

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