Your hibiscus is stressed and needs every bit of leafy canopy it has right now, so don't prune it. First, check the planting depth and make sure you didn't go any deeper than the plant was when in the pot. If it is deeper, the plant would benefit from being lifted and replanted. Next, apply a layer of compost to the soil around the plant. If you've mulched it, rake the mulch off, apply 3 inches of a lightweight compost (the kind sold by volume, not weight), and then put a 2-3-inch layer of mulch on top. Extend your mulch ring out to the tree's dripline if you can. These layers over the ground will help keep soil temperatures cooler in the hot summer sun and aid the root system in establishing itself. The fertilizer was a good idea, but youve got to be careful to use fertilizer in moderation on newly planted trees. Avoid fertilizing again for a full year and next time you fertilize, find a balanced fertilizer that contains iron.