First of all, I would not be feeding a plant that is not doing well to begin with, it may make the situation worse. Secondly,it sounds as though your fern may not have been watered enough while outside in the heat. These plants excel in relatively small container, but smaller containers dry out really fast in the heat. Depending on conditions, it might require daily watering. Another potential problem is that these plants are usually grown in a very peaty mix or one with a lot of sphagnum moss. Sometimes when these soils dry, they don't really hold the water when you do water. You may think you're watering but the water is merely running out the bottom! If you think this is the case, I would literally submerge the plant in a bucket of water for short period (under a minute should do) until you stop seeing bubbles rising to the surface. Don't despair though, they are very resilient. I threw one into the compost pile last winter thinking it was dead and a few weeks weeks later it was sprouting new leaves (fortunately, it was a mild winter). Good luck!