Just throwing it in that you won't actually need Fertilizer to grow any berries. As long as you have a pre-existing berry, in your case an Oran Berry, and put it in the Berry Pots with some water, then you'll be set to go. Grow about five or more of them and feed them to the sick Miltank and it should get better. Talk to the girls after and you'll earn an item.
Next time, please use the HG/SS Help Thread for all questions, which you'll find as an available sticky to the top of the board. Seeing as you're question has been answered though, just going to close.