Bad Sushi at John Dory's in Mall of Mthatha

Yesterday I went to have sushi at John Dory's in Mall of Mthatha. I ordered the bean curd and the rainbow warrior. When I ate the sushi I discovered that the construction of sushi was faulty, it was a mess. The rice was overcooked, the avocado looked like it was smeared onto the sushi and there was too little fish compared to the rice. What made me to stop eating was that I noticed that the colour of the salmon in one of the pieces was not ok, it had discoloration. They had inserted pieces of salmon that were not fresh. I showed it to the waitress and the manager on duty and requested a discount on the sushi because I had eaten some of it. I was willing to pay for the drinks but requested a discount on the sushi. The manager informed me that he couldn't give me a discount because of John Dory's policies, even after I reminded him I had every right not to pay for the meal since they'd placed my health in jeopardy. Please advise me on what I could do because I fear that John Dory's in Mthatha is selling low quality sushi using fish that is not fresh. I took pictures of the sushi and the receipt of my purchase.