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I honestly don't know, but I would be worried that is was some sort of fungus.
Has anything happened that might have stressed them lately? I don't know why it's just them, unless they are more susceptible than other fish varieties and show it sooner.
Have you got another tank you could quarantine them in, and treat both tanks - theirs in the hope you'll get rid of whatever it is, and the main tank as a prophylactic measure to try to stop is spreading?
I'm sure that wiser heads will be along in a moment and someone will probably identify it and offer advice.
It's usually helpful if you can include water parameters eg pH, nitrate and nitrite levels etc, temperature - the more information the better, really. Have you added any new plants recently - anything that might be harbouring parasites or infections?
Yeah i see that thanks guys, my water readings have been checked every week and they were perfect 0 ammonia, 0 nitrite and light orange nitrate but today when the rainbows woke up like that we saw some ammonia and nitrate at 20 which is actually the third to last reading as im looking at the booklet and 0 nitrite still. Ph is always perfect.What Adorabelle is getting at is that any ammonia at all means something is wrong. So if you got a "very light green" reading that is still an ammonia reading and means you need to perform large daily water changes for several days. Ammonia should always be 0ppm. Typically you only will have problems with disease and illness when something is wrong in the tank (which can be many factors), it doesn't just randomly happen without cause. And an ammonia reading is probably the leading cause of sickness for fish. It's good that your nitrite reading is 0 though.
Also, you should be changing at least 25%, ideally 50-75%, of your water weekly - not every 2-3 weeks. That's probably the cause of your ammonia readings - water changes that are too small and infrequent.
@Colun_T f it was protozoan, what would we treat with?
Using Salt to Treat Fish Health Issues.
For some fish diseases you can use salt (sodium chloride) to treat the ailment rather than using a chemical based medication. Salt is relatively safe and is regularly used in the aquaculture industry to treat food fish for diseases. Salt has been successfully used to treat minor fungal and bacterial infections, as well as a number of external protozoan infections. Salt alone will not treat whitespot (Ichthyophthirius) or Velvet (Oodinium) but will treat most other types of protozoan infections in freshwater fishes.