could you help me polish my writing?thank you!

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This letter is to train my students' ability of complaining in English. would anyone can help me polish it? thank you.(I have posted another one in mylot, sorry to trouble you, my friends. ) Dear Sir, I'm writing to tell you about my unforgettable experience in your restaurant. This morning, I orderd fish, my favourate food, just as usual. What a shock that it tasted so terrible. There is no doubt it has gone bad for long time. To be honest, I can't imagine the restaurant I trust so much can treat its frequent customers in this unbelievable way? I guess you may not want to waste the smelly fish, or rather, waste money? However, how can you manage to make money at the cost of customers' health? What's worse, you only do as you like despite the law? Maybe it is not only a matter of hurting customers' heart. Looking forward for your reply. yours, Peter