Stainless steel roll film tanks

As part of buying out a friend's darkroom, got several stainless steel tanks for developing roll film. Everything seems to fit fine, but after the tank is full and I want to open the top quickly to dump it, the top frequently is stuck on and seems to take a sledge hammer and chisel to get it off. All while the timer is buzzing, of course. How can you make sure these things will come off quickly??

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