Hi there
I think I will mention first that sailfin plecs can grow up to 18 inches. Condsidering the volume of water your tank holds, I reckon it must be around 2ft long.
If you can accomodate and afford a bigger tank, I'd give it some serious thought. As your present tank will become too small for your plec over the next year.Infact, you will probably find yourself having to re-house your plec elsewhere in a year or two
It is a pity some LFS sell them like they do. They know how big they will eventually grow. And the space of time they put on enough growth to have to re-housed
The parasite (ichthyophthirius multifiliis) that causes Whitespot is something that is present in the tank all the time.As Julie has said, various things can cause it to become active. Often it is due to fluctuations in water temperature.Already infected fish that are introduced to the tank.Stressed fish are very prone to developing whitespot.
It can be carried to other tanks simply by not washing your hands properly, nets or anything else that might be used in more than one tank. So such equipment should be sterilised after use in an infected tank.
Also as Julie has said, it would be helpful to know what treatment you have been using.
Raising the water temp can help to kill of the parasite. But should be done slowly. Some say it can go as high as 30, but others say 28.
Removing any obvious stress causing factors. Such as bullying fish.Or discouraging people from tapping the glass when looking at the fish.
Not sure from what you are saying how often you do water changes. But ideally, it should be carried out weekly, or at least every two weeks.
Hope this helps. But please let us know which treatment you are using too.