Hi, we have an large outside pond which has 5 large established goldfish. over the summer we have had lots of babies . Unfortunately, one of the established fish developed "cotton wool" patches over his body. He's a shy thing and we couldn't manage to catch and isolate him. The local "expert" told us to add salt to the pond to cure the cotton wool patches. Luckily we didn't just throw all the salt in at one. We started adding the salt little by little but noticed that some of the babies were dying off. We stopped adding the salt, drained off about 10% of the original water and replaced it with conditioned rainwater. But the little ones are still dying off and the established fish are listless and have no appetite. They are skittish. Only plus is that the cotton wool has gone from the infected fish.

We are trying to work out whether it was the addition of the salt or if there is another water based problem. We have tested the water and its spot on ph wise.

Anyone got any ideas?? Thanks Marian

I think this 'expert' needs to get his facts right on diseases. Salt does work, however it can be downright dangerous and it wouldn't be my first resort. Salt should really only be used in a freshwater bath away from the pond, to reduce the impact on other aquatic pond inhabitants (i.e aquatic insects).

The 'cotton wool' growths you describe are the typical symptoms of a fungal infection. Infections of this type are usually caused by a water mould known as Saprolegnia, which is abound in all bodies of water in background populations. However, it only becomes a problem when the fish are stressed, providing a weakened host which the fungus can infect.

Stress can be caused by numerous factors, however I would pinpoint the cause on poor water quality. Inadequate water conditions aren't really characterized by a pH that is too low or too high, but rather elevated concentrations of ammonia and nitrite, the former of which is a product of fish metabolism and decomposing organic matter. Both ammonia and nitrite are toxic to fish, even in small amounts, and the toxicty of ammonia increases with pH and water temperature. I would suggest testing the water for ammonia and nitrite as the results could be quite interesting.

In terms of treatment, dosing the pond with a powerful anti-fungal medication such as eSHA Labs 2000 should work. Alternatively, you could try Malachite green. Be very careful with this substance as it is carcinogenic. Malachite green attacks the fungus by penetrating the cell membranes, interfering with respiratory and metabolic functions inside the cell.

For salt baths, any affected fish should be removed and placed in a bath (typically a sterile food-grade plastic container) with a dilute salt solution. The fish should be removed immediately at the first sign of distressed behaviour and placed back into freshwater.

Finally, be very careful when using rainwater. Unless you are filtering it through activated carbon, you run the risk of introducing airborne pollutants to the pond water. Ecologically, these pollutants could have adverse effects.