Adding fish to your marine tank

Taking the mayhem out of marine fish keeping


Adding Fish to your marine tank


Once your CUC have settled in and you are confident that your water parameters have started to settle you can start to think about adding your first fish.


Any responsible marine fish keeper will have already thought carefully about a stocking list and it’s now, when you’re starting to introduce fish that this becomes even more important.


Regardless of the type of tank you are going for there will be some fish on your stocking list that are more territorial than others. Not only do you need to consider introducing hardier fish first as your tank will still be going through changes as it settles down, you also need to consider introducing the more territorial fish last. If you don’t do this you may find that the territorial fish will stop you from introducing anything else by simply bulling it to death.


So you have decided which fish you want and the order in which you’re are going to put them in the tank but there are a few things you need to bear in mind.




Plan the process first


It’s always worth putting a bit of planning into your purchase, decide which fish you are going for and stick with it. If you are lucky enough to have a good marine shop with a wide variety of stock it is too easy to get mesmerised by a completely different fish as soon as you walk in there. If you know your stuff or the shop knows their stuff then this could be ok but you don’t want to make an impulse buy only to get home and realise that your new fish is not suitable for a new tank, not suitable for your tank or completely incompatible with the rest of your stock list.


Remember that being transported to a new tank is very stressful for any creature so it would benefit all parties if your acclimatising equipment was all ready to ensure your new inhabitant gets into your tank as quickly as possible.


Buying the fish


If you can always use a reputable marine fish shop, ideally with some staff dedicated purely to the marine side of things. There is a massive difference between the knowledge of staff in different shops so even if you ask the right questions about your purchase you may get different answers depending on where you go.


When browsing fish in the shop you may see that a number of the same species are kept together. Whilst some fish can be successfully kept in shoals such as Anthias, some cardinals and some damsels (particularly Allen’s damsels), not all the fish you see in groups can be kept together in the home aquarium. Fish are often very stressed in fish shops, so stressed that they are not comfortable enough to declare territories or worry about their neighbours getting too close. Once you have safely transferred your fish to your marine tank and it’s settled in it will be confident enough to chase away anything it’s not happy about including the same and similar looking species.


It’s always a good idea to make sure the fish you are buying is healthy and eating well, look for negative symptoms such as:


  • Pinched belly, underweight.
  • Any noticeable bones
  • Abnormal behaviour, lying on its side or at the surface gasping for air
  • Breathing heavily
  • Any lumps, abrasions or unusual marks
  • Cloudy or swollen eyes
  • Flicking against rocks or sand


Many people will ask to see a fish feeding before they commit to purchasing it. Some shops are happy to do this, others are more reluctant. Feeding the fish before transporting it in a bag is not necessarily a good idea, especially if it has a long way to travel before it reaches your tank. Fish tend to be able to deal with stress much better on an empty stomach than a full one. Not only that but a full fish is more likely to produce more waste in the transportation bag rather than a hungry one. If you really do want to see the fish feeding (which is always a good idea if you can) then consider asking to see it being fed then returning the following day for it, many shops are happy to reserve a fish for a small deposit.


It’s often worth asking about the conditions the fish is kept in, some fish shops dose their fish only tanks with copper to protect against disease, whilst this is fine for the fish it means you must be very careful not to let any of the shop water enter your tank (you probably wouldn’t want to do this anyway). Copper in any quantity is often fatal to invertebrates so make sure you keep it out of your display tank. Even if you are running a FOWLR you should still avoid contamination with copper as it will interfere with the bacteria populations you are relying on for your biological filtration.


Getting your fish home


The most common scenario is that your fish will be packed in a clear bag tied at the end, some of the larger fish shops will actually insert oxygen into the bag, particularly if you tell them you have a long way to travel. This bag is then usually put in a darker bag. It’s helpful to have either a polystyrene box or even a cardboard box lined with newspaper to help the bag stand upright and reduce the extent of heat loss during your journey home.


On the way home don’t be tempted to take the bag out and have a look at your new purchase, the fish will be stressed out enough without suddenly being confronted with broad daylight and a face peering in at it. Try and get home as soon as you can, every minute the fish is in the bag the water quality will be deteriorating more and more and it doesn’t take much time before ammonia and nitrite levels will start to rise.


Whenever possible you should consider introducing your fish to a quarantine tank for a few weeks before putting it in your main display tank (see here for how to set a simple quarantine tank up). This gives your new fish time to relax before being introduced into a tank with other fish that may chase it for a while before it settles down. The most important thing about a quarantine tank, however, is it gives you time to assess the health of the fish and make sure it’s not going to introduce some sort of disease into your main display tank. Many fish diseases are fatal and require harsh treatments to eliminate them. These treatments are often copper based and as described above the copper will kill half of your display tank so infected fish will have to be treated in a separate tank anyway. It is much safer to quarantine your new arrival in the first place rather than rush around desperately trying to botch one together when the first sign of disease starts to show itself.


Under normal circumstances where the fish has been in the bag for an hour or less I would recommend acclimatising for around 30 minutes. It’s less essential to use the drip method (as mentioned here) with fish but still necessary to get them used to their new water relatively slowly. I tend to use a turkey baster, one squirt out of the bag, one squirt of tank water in. As soon as you get back open the bag immediately to get some fresh oxygen in there, turn the tank lights out (calms the other fish down and reduces stress on the new fish) and float the bag in the tank for ten minutes before you start to exchange water, a clothes peg can be handy for this to peg the bag to the side of the tank or a brace bar. As mentioned earlier make sure you discard the bag water as the quality will have deteriorated considerably.


If there is no way you can use a quarantine tank then the acclimatisation method remains the same but obviously you are taking a risk as you could be introducing a diseased fish into your system.


Leave the marine tank lights off for a few hours to let the new addition settle down, some people will actually leave them off for the rest of the day but different people have different opinions. I always turn the lights back on after a few hours and feed the tank just before the lights go off for the night. I tend to like doing this to check how well the new addition has settled in and see if it’s feeding.

Helpful tips


Time for fish!


Stick to your stocking list, decide what fish you’re going for and don’t get side tracked. You might find you come back with a fish that is incompatible with the rest of your stock list if you haven’t done your research first.


Try and get the fish back as quickly as possible, the longer it’s in the bag the more the water quality will deteriorate.

Marine fish - in a nutshell

When you can always use a separate quarantine tank, although it’s frustrating waiting before you can see your new addition in your display tank ti will be well worth it in the long run, it's heart breaking when you realise a new addition has introduced a disease to your main display tank and put your other marine tank inhabitants at risk

Try to stick to the stock list you have already researched, impulse buys can cause all sorts of problems and cause additional stress both for you and your marine tank inhabitants.

Try to do a bit of planning before your new purchase, get your acclimatising kit ready so when you get home you can start straight away

Turn your tank lights out before floating the fish bag to equalise temperature. Chances are the fish has travelled back in relative dark so to suddenly expose it to a bank of T5's or halides will increase stress levels considerably

Just because you see the same species of fish in groups in the fish shop doesn't mean you can always keep themin groups at home, the stress fish are under in fish shops often means they are not in the mood for holding territories


Always check for any symptoms of ill health in the shop, one of the most obvious is flicking themselves against the rock or sand which could be the start of any number of awful diseases



Remember that fish shops often dose their fish only tanks with copper, make sure none of this water enters your marine aquarium as copper based medications are often fatal to many invertebrates including the bacteria you rely on for biological filtration