I’ve Put A New Fish In The Aquarium, What Should I Look For?

Buying a new fish for the aquarium is without doubt exciting. We don’t keep aquariums just to look at a glass box! Even in a reef system fish make the scenery look more realistic and add movement. 

It’s assumed that all the correct actions have been completed before the moment of purchase. Above all else, the fish shouldn’t be an impulse buy, an act more likely to occur with less experienced aquarists. Obtaining a fish because it’s so pretty is wrong. The fish general type should be reasonably researched, that is it should be suited to the type of environment it is intended for (reef or fish only) and be compatible with the fish and corals, if any, already present or intended. The fish may look lovely but what is its final size? Does it have predatory tendencies? Particularly in a fish only system, but also for a reef the fish type’s character should be carefully judged as timid fish will not do so well among other bolder and more aggressive ones. Some research as above is important as once the local fish store is reached the fish on display, some very tempting because of their beauty, are likely to cause confusion. 

Fish for sale at a local fish store cannot be automatically accepted as being healthy. Some stores properly quarantine incoming fish and this has to be applauded. It’s necessary to ensure any quarantine has been properly done though – I learned of one store that did genuinely keep fish for two weeks before sale and then sold them. The problem was that the fish were not isolated for the two weeks in quarantine as new incoming fish were placed in the same tank. This exposed the existing fish to any incoming problem and each time a new fish went in the quarantine period should have been extended for a further two weeks (or more) but this didn’t happen. 

It’s important for the aquarist to question the health of any fish desired – spend time looking carefully at the fish for negative signs such as cloudy eyes, ragged fins, over fast respiration, hanging about listlessly in one area, spots and marks on the body etc. This is quite difficult for the inexperienced as for example fish have naturally differing rates of respiration. For example, a damsel has a faster gill beat than an angel. Also in a dealer’s tank the situation is far from perfect as the dealer needs to catch the fish on display quite quickly and a heavily decorated tank would seriously interfere with this. Nevertheless there are signs that can be spotted. If the store has a good reputation the assistant will be happy to explain anything. Once the aquarist is happy that all is well, ask to see the fish feed and again the assistant should be happy to oblige. If all is well ask the assistant what is usually fed – once home use this diet to start with so the fish recognizes the food if the species is known to be a bit finicky. The diet can be varied as time progresses. 

In the photo is a beautiful fish commonly called a copperband butterflyfish, the proper name is Chelmon rostratus. This is a reasonable example of what the less experienced aquarist should avoid. The fish though a ‘butterfly’ is generally suited to a reef aquarium but not a fish only system unless the fish are peaceful and even so food could be lost to other fish. A reef is best as the fish can pick at the rocks for food as it would in the wild, hence the long snout, though it’s unlikely the fish would find sufficient to eat. Even with a healthy fish the problem arises with standard feeding, with even experienced aquarists finding that they don’t feed. What is really frustrating is that sometimes they do feed without any problem – it seems to be a bit of a lottery. The one I owned lived in a reef system but I had to place food in a special rock with holes drilled in it so the fish could take it ‘naturally’. 

So the aquarist has picked the fish which is compatible and as far as can be ascertained healthy. It needs to be transported home and if not already quarantined put into a quarantine tank. Most aquarists don’t do this unfortunately. Then after a fortnight or more it should be properly introduced to the display aquarium, with dimmed light and time for the seawater condition between the quarantine tank or travel bag and the display aquarium to equalize. 

The first point when the fish is in the display aquarium is to watch for it to appear. Most fish tend to disappear into the rocks, or find a suitable place which offers some security. For all the fish knows there are predators about. Feeding the fish already present should continue as usual. The new fish will stay hidden for a while, the period very much dependant on the species and the company it has to keep. A healthy fish will re-appear and explore the aquarium. If the aquarist has provided sufficient hide-holes one of these will be adopted giving the fish more permanent security. Security is very important to the new fish as it has been stressed a great deal in the previous weeks with capture (unless tank-raised) and travel, so it needs time to calm down and resume its normal life patterns as far as possible. 

Once the fish has re-appeared a check needs to be made that it’s feeding. Some fish are bold feeders going for the food the instant it hits the seawater. Others are slower and some are timid, particularly if they are being harassed. The character of the fish should be taken into account to ensure it has enough food. Sometimes the fact that other fish are eating is sufficient for the new fish to join in. 

Whether the fish has been quarantined or not, once the fish is visible a watch should be kept for any changes in appearance or habits. If the way it swims changes, or breathing rates increase, or there are ragged fins it should be noted. If very small or tiny yellowish white to white spots are noticed on the fins or body then very close monitoring is needed. If the fish gives the opportunity have a look with a magnifying glass. Spots could be a sign of serious disease which if left alone could put other fish in danger. If such a sign appears don’t take immediate action but try to be sure that what is being medicated is actually there! Speedy action is necessary but there is time to consider. With some diseases it is easier to treat a fish only system than a reef. 

All aquarists keep a general eye on their fish to be sure they are healthy even if the fish have been resident for months or years.  After a time it becomes automatic at feeding and maintenance times. 

If the aquarist keeps high quality seawater and does the necessary ongoing maintenance plus feeds an adequate diet, with a little bit of luck the odds seem to be that the aquarium fish will settle and live a long healthy life. As experience grows the likelihood of being tempted to add ‘one more’ fish, possibly an inappropriate one, will diminish.



  1. I like your post, hopefully for more information like that.
    Where you get those beautiful photo? are they free of charge?

  2. Hello Dayan. Glad you liked the post. I often use photos of my own reef system, either in general or fish or corals. There’s no way I can obtain everything that way though so I search the well known free photo sites for one, which is where the copperband butterfly image came from.

  3. Great website! I am glad I found you guys. I have had a 75 gallon up and running with live rock and coral for over a year, added a bag of copepods once. How often would I have to purchase copepods if I wanted to get a mandarin goby? I hate to see fish die, especially because of my ignorance.

  4. Hello Josh.
    Glad you like the site. Your system should now be more or less properly mature, that is, over and above the initial maturation though full maturation can take a surprising time. I assume it’s a reef from your desription.
    The mandarin is considered by very many – correctly in my opinion – as one of the more ‘difficult’ fish, that is, to be kept by the more experienced aquarist. However, time doesn’t always produce educated experience, it depends on the aptitude/attitude of the aquarist! It’s also sometimes said that mandarins should only be kept in large reef tanks that have been running for years. I realize why this is (food supply and stable conditions) but don’t entirely agree. A controlling factor for all life is the availability of suitable food, and another is good environmental conditions. These two requirements need to be met.
    The mandarin is a specialist feeder as you clearly realize. They are lovely and that’s why many are doomed at purchase, the aquarist has bought on impulse – so sad. You haven’t and so there should be a reasonable chance of success.
    How often to add copepods is quite difficult. First of all, is there plenty of life in the purchase? Think of live brine shrimp, some shops sell a very ‘full of life’ bag and others, well, the shrimp could be counted no trouble at all! Also is there other life in the system that would be quite happy to eat copepods if they could – if there is then the supply period will clearly diminish.
    So it’s necessary to add food and monitor the state of the fish as is done with others, it should be clearly well fed and healthy. To assist with this, putting a little too much food in to start with shouldn’t do any harm so try what seems to you too much (within reason) and if all is well reduce the input over a period so the fish can indicate the situation. It’s a little hard to describe really.
    On a similar principle I fed my deep sand bed small pieces of fish to maintain the live population. At first I put a very small piece of fish every 6 ins or so and kept watch, the life was increasing. I continued but decreased the food slowly and noted no change in the population of beasties (as far as I could tell!) so I decreased the food which eventually fell to much less than half of the original amount. This feeding of bits of fish is more likely to pollute.
    Perhaps for a start put in one ‘dose’ of copepods. You should be able to see the fish eat over a period. A week later put in another ‘dose’. Continue to observe and if all seems well delay the next ‘dose’ to a fortnight later. If all continues well delay the ‘dose’ for a further week, making it three weeks. This would probably be the least to feed. This isn’t an instruction just a starting guideline! Observation is essential and it needs to continue.

  5. Thanks for the input. Considering the price of copepods I think I will try a mated pair of firefish instead. Keep up the good work.

  6. Best of luck – firefish are also lovely and interesting.

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