I only set up the tank less than 2 months. found the first white spot a week ago on one of the swordtails, but didn't take it seriously (bad decision!). Now, that swordtail who oringinally got it looks perfectly OK, but 5 guppies got it (2 of them just died yesterday ). Got a thing called "White Spot Cure" from LFS the day before yesterday. the instruction on the label is 2 drops per liter. It says one dose is enough, but 3 doses maximum, unless there's a full water change between the 3rd and the 4th dose.
first of all, I think it's very easy to over-dose. Here's why: I've got a 75 liter tank, so according to the instruction, I should put 150 drops. Wrong!!! It's a 75 liter tank alright, but I've got about 2 inch gravel and 2 big rocks, plus it's heavy planted, so really, there's only about 60 liter water in the tank. So I could have over-dosed by 25%!!! no wonder when I search the internet, lot of people don't trust chemical as they had the experience of killing all the fish in the tank.
I did lot of search on the internet, and lots of articles suggest the instructions on the label could be as much as 250% over dosed. So this was what I did: on day one, i put 60 drops (half of the dose suggested), on day two, I did a vacuum together with 10% water change and added another 30 drops. I'm day 3 at the moment. I've noticed that those 3 guppies that got the spots now have less on them!!!
I'm planning to do another vacuum tomorrow and put another 90 drops in will keep you updated on how I go.