Have the black spots been there since you had the fish? I assume that this is a new gotten fish, as in you have not had the fish for months.
Could be OB type markings. Sometimes pond raised fish have black spot parasites that eventually disappear since they don't reproduce inside aquariums. In any event, I would not get carried away with medicines and isolation, you may do harm and little good.
May not be a pure Red Zebra, but a hybrid more like an OB Peacock hybrid.
What do you suggest i do? The only other fish that has it is the other red zebra but only has one and not spreading. The one that i taken out looks like its increasing all over! Both fish a relatively new maybe around a month.
I would do nothing...most red zebras sold these days are pond raised and there is a parasite that causes the spots. Not contagious and I've never had a fish show distress with the spots. They go away in several months to a year.
I did try some remedies to no avail, waste of money.
The reason it's not contagious and can't spread is that the parasite requires several other hosts to mature, including a heron.