White spot is the worst issue an aquarist has to deal with. Stop parasite seems to work in a tank ONCE. The tank seems to become immune after the first dose.
If you have never dosed it before it will more than likely work. This is from experience and from reading other peoples success/failure stories.
If you have used it before in your tank, it probably wont work again.
MY THEORY:
White spot is EXTREMELY difficulkt to control and I believe you will never cure it until you STOP adding fish.
Since aquarists are generally always adding fish, (Due to not having finished their setup, or due to replacing fish) White spot canmnot be controlled.
White spot will vanish from a system only when you stop adding fish to it.
You may be lucky and add fish without WS, but it is more common to add it, especially with clowns and tangs and a few other species. WS cannot be seen in the early stages and fish that are already immune will be carriers, but will not show ANY signs.
MY ADVICE:
Forget quarantining fish unless you are prepared to do it for a MINIMUM of 6 weeks. If so, you will probably be fine.
THE REALISTIC WAY.
Add fish no more than 2 at a time and wait at least 3 months between doing so. This will allow ALL fish within the tank to acclimate to the WS and battle it at their own pace. After this 3 month period, ALL the fish that are still alive will be immune and good strong fish. The ones that die were never strong enough for your system anyway.
TWO STEPS FORWARD, HALF A STEP BACK.
If you are wanting to add very delicate fish such as WS prone tangs such as Powder blues, Browns and Clown tangs etc, It should be designed that these are the LAST fishes to be added and if adding more than one of these delicate fishes, they should be added together. It would also be advisable to get them from the same supplier that have been in the same tank water as each other. This ensures they will not give each other WS if from seperate systems.
Once these delicate fishes are added, it is possible to kill them very quickly by adding other fish. If you add another fish with WS, (visable or not), the new delicate additions will rapidly develop WS and more than likely die very quickly.
In the ocean, WS can be diluted within seconds due to the mere size of it. Once in you tank, you are screwed until immunity takes place.
THE ONLY SUREFIRE WAY TO REMOVE IT IS THROUGH IMMUNITY.
The addition of garlic may help with speeding up immunity also, but will not CURE it.
I have lost nearly 12 fish in all due to WS and have learned by experience.
I hope this helps.
Craig