So you have set up your fish tank and equipped yourself with enough knowledge in the world of fish keeping. You have the pet fish you wanted and the beautiful aquascape created for them. But in the management of your fish, it is important to know every sign to look for if they are sick.
Fish disease or fish stress can be any abnormal behavior or changes externally observed in the fish. There are internal signs as well which can be determined by dissection under anesthesia or of a dead fish. Illness-causing organisms and parasites rapidly grow in a closed environment such as a fish tank and daily tank checks should be done by fish keepers.
In your daily tank checks, closely observe the fish. Be it in freshwater or saltwater tanks, there are signs that could indicate a possible problem and could be caused by a number of reasons. Here is a list of fish disease signs that you should watch out for.

sick-fish
Firstly, the tank signs that could point present or future problems with the fish are by far, the most easily observable. Cloudy water may indicate presence of ammonia, bacteria, algae bloom, parasites, decaying organics or the air and filter is off. There may be protein foams in the surface of the water which could mean that the tank wasn’t washed well.
You fish tank may also have a hole exposed to under gravel filter allowing the water to bypass substrates, decreasing biological filtration and causing a sick fish. There may be dead specimens and uneaten food in the tank causing it to foul. This could lead to poor disease control.
When it comes to behavior, a sick fish may show violent reactions such as thrashing and darting. They may stay in just one place and move side to side, scratch on objects in the tank, try to jump out and the normally bottom fish would now be on the surface or the normally surface fish would now be at the bottom. These movements are all signs of a decreased equilibrium.
Check the fish for gasping, squirting water and if the mouth is not closed. The opercules may be flared or irregularly beating. These breathing, opercular and gill problems usually mean a parasitic and bacterial infestation.
If your pet fish is not eating, it may indicate presence of bacteria, parasites and toxins or liver damage or internal injury. But if eating and still losing weight, the most probable cause would be the presence of toxins. Their eating problems are usually resulting in liver damage.
Some external signs to be wary of clamped fins, sunken or bloated belly, white or dark spots on the skin, fish tissue extensions or bloody spots or streaks. A sick fish could also have fight injuries because of its violent behavioral changes. They could be experiencing an increased mucus production or having visible living objects on the skin such as copepods which are parasitic.
For the eyes, it may be cloudy, red or bloody, protruding or sunken. There may be color changes in a sick fish such as the gills turning pale and abnormal growths in the skin. Hopefully, you won’t find your fish having any of these signs so check your fish tank daily to prevent a sick fish.