If you have the chrome extension installed and you are logged in you can right click somewhere in chrome. In the appearing menu select “link.fish”. One of the points in there will be “Set active collection” where you can choose which one should be active.
The collection which is set there will be the one new pages will get saved to. Depending on that will also the link.fish-icon change when you are on a page which got already saved in the selected collection.