Not really that hard, but it is a pain. I resealed an old 29 gallon and it holds water just fine,the new seams look pretty bad though ( not acceptable for a display). If you do decide to redo the seams make sure you get the right silicone, GE Silicone I (the one without bioguard) or something specific to aquariums. The smell of curing silicone is really bad.