Please help me, quickly. Clown loach has developed white patches in just a few hours. (Not Ich)?

The most likely causes are either parasites (Costia or Chilodonella) or Columnaris. Since you don't know for sure, the best plan of action would be to treat with something that would be effective against both-- for this, I'd either use Coppersafe, General Cure, Aquarisol, Quick Cure, Organicure, Anti Fungus, Aquasol, Cure Ich, Ich Clear, or Parasite Clear.