I have kept cacti for many years. Each year I take cuttings, plant them and watch them grow steadily over the rears. I use my own fertiliser – comfrey and nettles soaked in water for weeks. It smells but my goodness, it makes the cacti grow, The secret is not to over-water.  Taking  care of a banknote collection  requires the same dedication and interest. You keep looking for notes to add to the collection and like the cacti, the collection grows steadily over the years. Patience, a keen determination to buy the right note, all this is key to developing a very decent collection.