Welcome, murmaid.
Staghorn ferns are epiphytic plants, and though I will sometimes use Miracle-Gro (diluted at 1/4 the recommended rate in the cool/cold months and at 1/2 the rate in the warm/hot months), I typically use my "Orchid" fertilizer on them. Since Staghorns don't flower (they propagate via spores), I use a balanced orchid fertilizer, 20-20-20. I fertilize my tropical plants all year long and since I am mostly using orchid fertilizers, I dilute my fertilizer at 1/2 the recommended rate (usually that means 1/2 tsp. per gallon). I spray the leaves with this diluted fertilizer way more than I actually "water" the plant.
Is/are your staghorn mounted, growing in a pot(s), or in a basket(s)? What is your watering/fertilization schedule?
Any big-box store will have orchid fertilizer.
