Since you only have a limited number of plasmid and tonic slots available for splicing, you're going to need to make at least a few visits to Rapture's Gene Banks. These fee-free machines (courtesy of Ryan Industries) allow you to swap any of your currently assigned plasmids or tonics with any others you've picked up during your travels. It's a nice service to have available when you're lacking the proper plasmid or tonic to make it through a specific obstacle.