One red cap oranda fish should only be in that tank, you are overstocked, you will have to see if the lfs will take the others back if you wan't to keep the blackmoor with the red cap you will have to get a bigger tank in the near future, goldfish are big waste producers and need an excellent filter to cope with there waste.
The rule for goldfish is one first goldfish 20gal and then 10gal for another one.