It sounds like the life cycle of the white spots hasn't had time to be destroyed completely, causing your fish to keep catching the illness repeatedly. When treating the fish, you should leave the salt in there for 7 days straight, regardless if your fish look better or not. That should give them enough time to recover completely. And if you do a water change, replace the ammount of salt you took out. I recommend using 3 tablespoons of salt for every 10 gallons of water you have. Goldfish really can tolerate an astounding ammount of salt, so don't be afraid to use the full dosage! For instance, if you have a 20 gallon tank you should add 6 tablespoons of salt. If you do a 50% water change, you should add 3 tablespoons to replace the salt you removed. If you don't replace the salt it will dilute the concentration you have in there. Fin rot is something you really have to nip in the bud right away; else your fish end up loosing their entire tails to the disease. For a small ammount of time it will be fine to use the salt. You just don't want to keep it in there all the time weeks after they have recovered.  

Hope this helps!  

I would think every other day. But check with Goldiegirl too because she knows more than I do.

Thanks for the great advice, I will try your suggestions, if they dont clear up and stay free of it, I will probably be posting asking about the best medications to use

Sorry I was late on this post...been really busy the past few days! The salt should pretty much do the trick (that stuff works wonders), however sometimes stronger meds are needed. I would just suggest keepiing a very close watch on  them and try to avoid stressing them as much as possible (i.e. chasing them around with the net, bumping into the tank...). Keep the water perfect...I would suggest testing it often so you know for certain. Nossie's advice about the heater is also great - warmer temperatures make the life cycle speed up, destroying them faster.

Good luck with your fish and keep us posted. 

"Separate Container" in that case has to mean a "cycled quarantine tank" to avoid causing any damage to fish by moving it...
It could be a plastic container of sufficient size, (10 gallons - 40 liters) but since you already have the parasites obviously in the tank, it would really be the best solution to just treat all of it. Just salt won't have an impact on the eco-system of the tank either way

Yes, with ich it is important to syphon the bottom of the tank as well due to the life cycle of the ich cysts...

When the ich protozoan attaches to the side of the fish, it literally begins to eat the fish alive. The fish usually produces extra slime coating to try to protect itself. The ich protozoan continues feeding and growing. This is why ich can be hard to treat with meds - the cyst wall prevents anything from penatrating it and harming the ich protozoan itself. Once it matures, it will burst through the cyst wall and then fall to the bottom of the aquarium into the gravel. It then begins to divide into hundreds of new ich-infecting "baby ichs." At this stage the temperature influences the ich most, the warmer it is the faster it multiplies. At temps around 78 degrees F, the replication will be completed in about 8 hours. At lower temperatures, the replication takes longer making the treatment time for eradication much longer.

Once the replication is complete, the ich protozoan bursts and releases the newly-formed ones into the water. These are mobile and swim around the tank searching for a fish to attack. Once they attach to a fish, the cycle will start over again. It is during this stage that ich is most susceptible to treatment. These protozoans go all over the tank, which is why it's important not to transfer gravel, plants, or other decor to another tank unless they are first sterilize. Depending on the temperature of the tank, it can take anywhere from 4 days to several weeks to get rid of all these horrible little monsters.