If a cat eats fried fish, it may experience symptoms such as diarrhea, loss of appetite, or vomiting. These symptoms are usually due to the fried food putting a strain on the cat's digestive system, leading to indigestion or gastrointestinal upset. Methods to address this include:

Observe your cat's symptoms: First, closely observe your cat's symptoms, such as diarrhea, decreased appetite, and vomiting. These symptoms may persist for a while, but usually resolve on their own.
Offer easily digestible food: When your cat is unwell, offer easily digestible food such as wet food or cooked chicken breast to help reduce the burden on its digestive system.
Replenish fluids: Ensure your cat has adequate water intake to prevent dehydration. Try offering small amounts of water frequently, or provide wet food to ensure adequate hydration and nutrition.
Using probiotics: Giving your cat probiotics can help regulate their gut flora, promote digestion, and relieve symptoms such as diarrhea.