With only 100L I wouldn't add anything, and definitely upgrade soon. For some reason I thought I remembered you having something like 50 gallons, but you have half that... Mumma is right, the tank is overstocked as it is now. Also, the stocking at the LFS
is not usually a good example of proper stocking. However, if you want to see more of your knife you could add a large piece of clear tubing for him to hide in. It will make him more comfortable but you will still be able to see him. And your tank hasn't finished cycling? Keep a good eye on him, I don't think ghost knifes are good fish to cycle with since they are scaleless. Scaleless fish are usually more sensitive to water parameters.