How do I get the white spot off acrylic tank

If your tank is acrylic. acetone is the last cleaner to use. It softens certain plastics. As you found out, it most likely softened plastic while wiping it transferred material onto the clear plastic now discolored. Drying may leave stains, marks and cloudy spots. Once sanded down, the only way to restore clarity would be using vapor deposition of......acetone as it can restore clarity. Search for headlight lens vapor deposition using acetone vapors to restore headlights made of similar plastic. Here's one link; https://www.youtube.com/watch?v=mNf49ldX...