Yes, unfortunately you can marinate tuna steaks for too long. While most meats live under the golden rule of marinating - that the longer the better and more flavourful - that is simply not the case with tuna steaks. If tuna steaks sit in a marinade for too long, the acidity in that marinade will start to break it down and ruin the texture of the fish. Tuna steaks should marinate anywhere from 30 minutes to 2 hours for best results.