Hello, this is parasite named ICK, this can be from dirty gravel to plant, fish infected with it, that is why whenever you are going to buy a fish or plant see all the fish if there is something wrong
That happened to me last month and I treated mine with salt and it gone
NOTE
: I used table salt, just verify it does not have iodide and it is coarse crystal not crushed like these
And here are the directions:
In case of serious infection can add 3 tablespoons of salt per 4 liters at once (this can be dangerous for plants). After treatment, the salt is removed by successive changes of water for 5 to 10 days. We may also use for disease prevention at the rate of 1 "teaspoon" of salt per 20 liters of aquarium.
Salt is a drug running slow (20 days for total elimination of germs), respecting the cycle of bacterial filters (if added gradually), used for most species of fish, cheap, does not weaken the fish sick, is not toxic for the handling, not attack aquarium boards, hard on metering, measurable (by specific tests), and does not deteriorate with light and with time. It is ideal for treating the aquarium ovoviviparous (guppies, platys, mollies, sword tails) and goldfish and koi carp (both in the aquarium and in the pond). With these data should not hesitate to use it when and where needed.
You might say that This is a big amount of salt but I used on my 10g fish tank and in about 15 days they were gone
The white spots will begin to disappear in about 3-5 days
Hope I helped You, Send me a message if you have questions