Need a better picture.
If it's white and fluffy then it's fungus. Fungus is a secondary infection that gets into open wounds.
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A broad spectrum fish medication that treats fungus, bacteria and protozoans might help. But you need to find the underlying cause of the wound so it doesn't happen again.
You can also try salt. Add 2 heaped tablespoons of rock salt, sea salt or swimming pool salt for every 20 litres of tank water. Keep the salt levels up for 2 weeks and see if it helps. You can use salt and medications together if you need to.
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Before you treat the tank, do the following things.
Work out the volume of water in the tank:
measure length x width x height in cm.
divide by 1000.
= volume in litres.
When you measure the height, measure from the top of the substrate to the top of the water level.
There is a calculator/ converter in the "FishForum.net Calculator" under "Useful Links" at the bottom of this page that will let you convert litres to gallons if you need it.
Remove carbon from the filter before treating or it will adsorb the medication and stop it working.
Wipe the inside of the glass down with a clean fish sponge.
Do a 75% water change and gravel clean the substrate. The water changes and gravel cleaning will reduce the number of disease organisms in the water and provide a cleaner environment for the fish to recover in.
Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
Clean the filter if it hasn't been done in the last 2 weeks. However, if the filter is less than 6 weeks old, do not clean it. Wash filter materials/ media in a bucket of tank water.
Increase surface turbulence/ aeration when using medications because they reduce the dissolved oxygen in the water.
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The following link has information about what to do if your fish get sick. It's long and boring but worth a read when you have some spare time.
If your fish ever looks sick or unwell, then the following steps might help. Test the Water and Clean the Tank. Test the water quality for ammonia, nitrite, nitrate and pH and write the results down in numbers. Check it for general hardness (GH) and carbonate hardness (KH) too if you can, but...
www.fishforums.net