Fish are desperately ill

Question
Hi Carlo,
Two of our fish have been moving very little, are loosing their colouring but most of all have bad swelling to their undersides.
One of them has swelling to its right eye and both of them are now just floating and not able to move. One of them is however moving its eyes and mouth.
Do you know what this could be or have any advice?
Thank you,
James

Answer
Hi James,

First of all the fish should be removed from the place where they have been kept and put into a hospital tank.
Make sure that the water temperature and ph are the same.

They are probably infested by a parasite called Costia.

Costia usually appears where the water in not being properly cleaned.

A salt bath would eliminate this parasite.
Use one kilogram of salt per 100 liters of water.
Keep the fish in this solution for no more that 12 days.
The hospital tank should have a proper filtering and oxygenating system.

It would do no harm to do the same to all other fish.

Adding 3 kilograms of salt to every 100 liters of water to the pond
WITH NO FISH IN IT! and letting it sit for a couple of days would also help disinfecting it.

Good luck,

Carlo