Large and medium size Snake Plants (2) #freecycle

Months ago I bought the large snake plant at ShopRite (big mistake to buy there) and it had fungus knats. It infected the smaller plant as well. I have changed the soil on both twice yet the knats return. I give up and am ready to trash them unless somebody has the patience to de-knat them. Despite the knats, the plants look healthy and have grown twice their size since I got them.