Signs That You Might Have Stressed Fish: The Symptoms


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This thread will try to list all the signs that indicate a fish might be stressed. Hopefully this will help catch a disease early or give you an indication that there is something wrong in the tank and allow you some time to react before it is too late. Probably also let you know that the fish is sensitive to the medication or needs more observation while in quarantine.

When I started the hobby I always heard statements like if fish is stressed do this or that, but I didn’t really know what signs to look for to indicate that the fish is really stressed. Looking back now I can remember sometimes fish being stressed and I though it was normal behavior, that’s why I hope this thread will give you a heads up and enough time to react to give a better environment for the fish. It can be really tricky to spot stressed fish as they are really good at masking the signs (for survival reasons), so constant observation is key in spotting it early.

Symptoms to look for in stressed fish


Fish spend all their time swimming, that’s what they do. You want to research your fish, probably watch some videos of a healthy fish and observe their swimming behaviors. Each fish is unique and will have different swimming behavior or patterns than other fish. For example, tangs will usually spend their time swimming in the open going back and forth through the tank, on the other hand, clown fish will spend their time swimming in small section of the tank and will usually only swim a foot away from that location. Other fish have different behaviors.

If the fish started darting or hysterically swimming in the water, then this might indicate a recent problem in aquarium or beginning of a disease. On the other hand, if the fish seems lethargic and its swimming acivity decreased this might indicate a chronic problem in the tank. Note that lethargy can indicate a parasite too, so make sure to check for other symptoms.

If the fish is laying on its side or trying to reach the top and gasping it sometimes indicate that the water parameters are out of the norm. It can be that there is a build up of ammonia or nitrite (or nitrate if they are really high), it can also be due to the lack of oxygen. Sometimes low/high ph, temperature, or salinity might cause similar effects. So make sure your parameters are within the required range for the fish to reduce the stress.


One of the easiest giveaway that a fish is stressed is its colors. Fish requires energy to maintain its colors. When its stressed, this energy is rather conserved for more important survival functions, so you will notice that the color is fading away or pale or that the fish is not as colorful as it once was. Also, the stress hormone Corticosteron will cause the colors to fade. Don’t mistake a pale skin or fading of colors with white spots caused from parasites like ich, stress will not cause ich to infect the fish out of nowhere except if it was there to begin with. If you see white spots on fish make sure to start treatment ASAP, and if you are unsure how to treat it always ask the reefsquad and they will be more than happy to help you.


Another important thing to observe is the fish’s appetite. For a fish to stay healthy it needs to eat, some diseases or sometimes stress will cause the fish to stop eating. This is true if you introduced an aggressive fish to the tank, where other fish might get stressed and start watching out for it instead of looking for food. You will need to make sure to get the fish to eat again as fast as possible since if it is not getting energy it can’t heal or continue fleeing the aggressive fish and will eventually cause is it to perish. Make sure you are feeding the right diet for the fish, some fish are herbivores, others are carnivores and then you have omnivores. Also some fish, like dragonnets, are very finicky eaters and in most cases will not accept any food except what they eat in nature. Feeding the wrong diet might stress the fish and cause it to develop diseases.


Similar to humans, when fish are stressed or anxious, the breathing rate will increase. This can also be a result of parasitic infection (also a stressor). Another factor that might affect breathing is bad water quality. It can indicate lack of enough oxygen in the tank and the fish is breathing faster to suck in enough oxygen to survive, or the presence of ammonia where the gills might be damaged and not as effective in gas exchange. In all previous cases the fish will be stressed and you need to find the right source of the stress.

Other Stress Factors


If the fish is infected or has an injury it will get stressed. Be sure to isolate the fish in QT to reduce the stress to a minimum. As the fish stress out, its immune system will weaken, resulting in secondary infections, which in turn will add more stress to the fish and so on. Make sure to medicate as necessary and watch out for other stress factors listed previously. Sometimes the fish might be able to heal on its own and will not show any other stress symptoms, so use your judgement to decide whether to treat it or not (this is the case only for wounds and minor bacterial infections, always treat for parasitic infections or severe bacterial infections).

Hiding Areas/ Schooling Fish

Another stress factor is lack of hiding spaces, which will leave the fish exposed to predators. Even if you have peaceful fish in the tank, fish like to have some hiding space to feel safe if threatened. Make sure you have enough hiding spaces for the fish, especially in quarantine. Another related issue is getting one or two schooling fish in the aquarium. If it is schooling fish make sure to search the minimum recommended fish to create a school. The fish use the school as protection by blending (hiding) in it decreasing the chance of getting hunted by predator.


Probably you already knew it as some people refuse to quarantine fish because they believe it will cause unnecessary stress. Unfortunately, we can’t measure the stress levels of the fish in quarantine but I believe quarantining the fish will reduce a lot of the stress from the fish on arrival. Yes, there will be some stress still involved, but not as much as adding it directly to DT (this topic is debatable and this is only my opinion), here is why: The fish can recover from transportation stress in the quarantine. It can heal up from lack of oxygen and ammonia build up due to being in a bag for the transportation journey, along with all the agitation from handling it. While in quarantine it can start eating without any competition from other fish. It gives you time to observe the fish and get to know it better. It allow you to watch for signs of diseases that might stress it and other fish if placed directly to DT. There are many other benefits that makes quarantining worth slight increased stress the fish will induce as a result of that. If quarantining is done correctly, it might also reduce the stress level to lower than that in DT, again this is all debatable as we can’t really measure the stress in fish, but if you think about it it is logical to assume so. Also, I noticed through TTM if both tanks were matched exactly, the fish will recover from transfer within few minutes.

You still need to ensure that the quarantine is big enough for the fish. You might get away with 5 gallon quarantine for small clownfish, but you don't want to add a tang to that for example. Always check the parameters in QT as in most cases parameters will go out of the norm easily as its temporary and didn't mature enough to have stable parameters, and sometimes it doesn't include biological filtration, such in the case of TTM.


The parameters in the tank will always fluctuate, whether it is temperature, ph, salinity, or other minor parameters. These fluctuations will cause stress to fish, although they are not major and will probably not effect it in the long run as sea water will also experience some fluctuation in its levels. What might cause fish to stress out in the tank though is getting the wrong size tank for the fish. With tanks it is always the bigger the better, sometimes fish will manage to live in small tanks, but there is a limit to what is considered too small. For example depending on the tang, the minimum tank size you want is 70 gallons, while for other fish like a clownfish it can be 30 gallons. Some fish require minimum of 250 gallons and so on. Make sure that your fish will fit your tank.

Another stress factor is overstocking the tank. Your fish might be able to live in your large tank but you have too many fish in there, this will result in stress where fish will start competing for food, territory, etc. along with the build up of waste and the deterioration of water quality and so on. After all remember that our tanks are considered overstocked when compared to nature. Fish densities completely vary from location to location, but the average seem to be around 1-2 fish per square meter, thats almost like having only 1 or 2 fish in tank (assuming its common size tank 40-200 gal tanks).

Finally, with all the equipments and electrical wiring running through water you will have stray voltage in water. In most cases this will be fine for fish, but sometimes it can get to dangerous levels for you and for the fish. Make sure you get high quality equipments with proper insulation to reduce the stray voltage to minimum. Avoid adding extra unnecessary electrical equipments to the tank.

Finally, always remember to spend some time daily observing your fish. Other than the fact that it’s beautiful and rewarding by itself, it will let you know immediately when something is wrong, you will quickly figure out that the behavior of the fish has changed. Also less stress will result in healthier, more beautiful fish. Quarantine and Tank stress are considered minor and hopefully will not effect the fish in the long run (except if they are exaggerated), but other stress factors like swimming behavior or breathing will require an action from you to find the problem as they are usually the result of something major hidden and waiting to breakthrough. The list is not complete and definitely there are more stress factors and Ill try my best to keep it updated.

And as always, Happy and Safe Reefing.


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Awesome thread and thank you!
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