Apply liquid fertilizer bi-weekly or monthly. You can use fertilizer when new fronds (leaves) are popping up.
You can use 3-1-2 fertilizer as directed on the package. But, when applying for the first time, we suggest you dilute your fertilizer by half the recommended amount on the package (this is to avoid plant burn).
Irrespective of the fertilizer you use, you need to have good organic nutrient-rich soil.
Too much fertilizer burns the leaves. Never feed maidenhair fern in the winter, late spring, or summer.