what many people don't realise is by the time the cysts are visible on the fish's skin, many medications are actually futile. Try increasing the temperature of your water to around 30°C. Also, please read this article, it will tell you everything you need to know about treating white spot, it's a very long article but please take your time reading it, it is probably one of the best articles on the Internet, not written by me I might add.
EDIT: always check the medication before putting it in your tank, like you have already suggested, some medications can harm plants, more importantly, they can have an effect on your biological filter. This is why quarantining fish is always advisable when possible.
www.oscarfishlover.com/index.php/ick/Tre...ite-Spot-or-Ick.html