Pan-frying fish filets with skin on
The tried and tested method for solving this is simple. As you add each fillet to a hot pan press down for 60 seconds then repeat as you add more fillets. If you are good you could do a fillet with each hand so as to make the process quicker.

Another really good method, and my favourite. Use a cold pan. Simply oil a decent non stick pan, put your fish in then put the pan on the heat as high as it'll go. You still get nice crisp skin but no curling.