Fine Net crafting recipe
The Fine Net is a tool (added in update 2.1.0) that is used to obtain Plankton and Krill. In order to craft it, you will need 4 sticks, 2 string, and one wool block.
Once you have the net, right-click with it on water until you obtain a Plankton or Krill item. Not all clicks will yield successful results, so be patient and keep trying. The net will gradually use up its durability as you use it.