If it's the white bit on the fin on the left hand side of the picture, then it is just fungus (Saprolegnia).
Fungus lives in the water all the time but only infects a fish when the fish gets damaged. This damage can be caused by an injury (scratch or bite) or from poor water quality damaging the fish's mucous coating and skin. Then the damaged area allows the fungus to get in and infect the fish. Cool water temperatures can also encourage fungal infections.
If none of the other fish are injured they probably won't develop it. But if the water isn't good, or they are breeding or fighting, then others might develop the same problem. Basically it can spread to everyone in the tank but is unlikely to do so unless the other fish are sick or injured as well.
Make sure the tank is clean, (regular water changes and gravel cleans), also clean the filter if it hasn’t been done in the last few weeks. Make sure the temperature is 24C or above. Then use a broad spectrum anti-bacterial/ anti-fungal medication. There are heaps of them around so you shouldn't have trouble finding one. You want a medication that treats bacteria as well as fungus because the fungal infection often allows bacteria into the wound as well.
Generally any medication containing Methylene Blue or Malachite Green should get rid of it.
Methylene Blue will make the silicon (glue holding the glass together) go blue and it will probably wipe out the filter bacteria so you should monitor the water quality for several weeks after you have treated them.
To work out the volume of water in the tank
measure Length x Width x Height in cm
divide by 1000
equals volume in litres
When measuring the height, measure from the top of the gravel to the top of the water level. If you have big rocks or driftwood in the tank, remove them before measuring the height.
Remove carbon from the filter before treating otherwise it will absorb the medication out of the water.
Fungus lives in the water all the time but only infects a fish when the fish gets damaged. This damage can be caused by an injury (scratch or bite) or from poor water quality damaging the fish's mucous coating and skin. Then the damaged area allows the fungus to get in and infect the fish. Cool water temperatures can also encourage fungal infections.
If none of the other fish are injured they probably won't develop it. But if the water isn't good, or they are breeding or fighting, then others might develop the same problem. Basically it can spread to everyone in the tank but is unlikely to do so unless the other fish are sick or injured as well.
Make sure the tank is clean, (regular water changes and gravel cleans), also clean the filter if it hasn’t been done in the last few weeks. Make sure the temperature is 24C or above. Then use a broad spectrum anti-bacterial/ anti-fungal medication. There are heaps of them around so you shouldn't have trouble finding one. You want a medication that treats bacteria as well as fungus because the fungal infection often allows bacteria into the wound as well.
Generally any medication containing Methylene Blue or Malachite Green should get rid of it.
Methylene Blue will make the silicon (glue holding the glass together) go blue and it will probably wipe out the filter bacteria so you should monitor the water quality for several weeks after you have treated them.
To work out the volume of water in the tank
measure Length x Width x Height in cm
divide by 1000
equals volume in litres
When measuring the height, measure from the top of the gravel to the top of the water level. If you have big rocks or driftwood in the tank, remove them before measuring the height.
Remove carbon from the filter before treating otherwise it will absorb the medication out of the water.

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