Does anyone know what this is and how to treat it? One has a white lump near its eye, the other some white stuff at the end of its fan tail. Both are usually in a pond with others, but are currently quarantined in separate buckets.
It looks like a fungal infection. It is common for pondfish to get this or other types of problems in the spring before the immune system of the fish starts working (it takes some time for the water to warm to a level where the immune system starts to function).
The common treatment for this is to add aquarium salt so the concentration is 0,3% of salt in the water, and to increase the temperature to 20 °C (68 °F).
If you are able to treat the fish in a separate tank you can add medication to the water; I use Tetra Medica General Tonic in addition to adding salt.
To limit the production of waste products, it is best to only feed very little food every 3 days; be sure the water is well oxygenated to limit the stress for your fish.
A salt concentration of 0,3% is not dangerous to goldfish or koi and can be continued indefinitely.
Adding medication to your pond is something I will advice against as it will destroy the biological balance in your pond.
1Thanks. These two are currently in a bucket each, which I've moved indoors so the temperature should be about 20C once they've warmed up to room temperature. Unfortunately I don't have any way to oxygenate the water in the buckets at the moment. I'll try and find some suitable plants, salt and treatment tomorrow but that might be hard while most places are still closed! Unfortunately we don't have enough spare in the pond atm.– RobApr 4, 2020 at 18:10
2to oxygenate the water you only need to get the surface water moving a little,a little fan blowing on the surface is fine. Apr 5, 2020 at 6:05
2I just remembered to come back to this thread! The top fish I treated for fungal infection in a bucket, it all cleared up and was back in the pond in about 3-4 weeks. The fan tail fish had a blossom petal or something on the end of its tail, that fell off the day after. All is now well, thanks for your help! :)– RobMay 21, 2020 at 15:13