Fish ulcers are caused by a specific bacteria generally due to unfavorable water conditions.
My recommended treatment is:
1) Change 25% of the water in the container making sure to use a dechlorinator that eliminates both chlorine and chloramines.
2) Add aquarium salt to the water at a ratio of 1 tablespoon per every 5 gallons/20 litres of water.
3) Make sure the water is kept in a temperature range of 62f-72f/ 16c-22c degrees.
Also try and keep the water at a steady temperature so that it does not fluctuate.
4) Begin to treat the fish with a medicine called Maracyn Plus, click on upcoming link for a view of the product, http://www.drsfostersmith.com/product/prod_display.cfm?pcatid=4833 , which is available at most major pet stores. Follow the directions on the product label for usage instructions.
Note: if the container where the fish are being housed holds more than 80 gallons of water or if you are located outside of the USA, please let me know and I'll recommend a different medicine.
Warm regards and please let me know if you have any questions.
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Maracyn Plus is not available in the UK so a good substitute medicine that is available in the UK is called Waterlife Myxazin, click on upcoming link for a view of the product.
http://www.seapets.co.uk/products/aquarium-supplies/fish-treatments/tropical/tropical-fish-medicines/waterlife-research-medicines/waterlife-myxazin-60ml.html .
As with all medicines make sure to follow the directions on the product label.
If the container holds a large amount of water it might be better to use a medicine called
Tetra Pond Medifin, click on upcoming link for a view of the product,
http://www.seapets.co.uk/products/pond-supplies/pond-treatments/fish-treatments-and-medicines/tetra-pond-medifin-250ml.html .
Reminder: Please make sure to change 25% of the water at least once a week using dechlorinated water that is the same temperature as the water already in the container.
YOUR TROPICAL FISH RESOURCE
OVER 20 YEARS EXPERIENCE W/TROPICAL FISH---BREEDING, MAINTAINING, CURING