Hi everyone. I am still fairly new to houseplants. I have a pink pelargonium that is in its second flush of blooms (I got it a few months ago). It was doing fine until I fertilized it with fish emulsion last week. (I used 1/8 tsp. per 4 quarts of water.) I don't know what kind of soil it is in, because I haven't repotted it since buying it from the nursery. The edges on the biggest leaves are turning bright yellow (like a halo). It is still blooming, and is not dying. This seems to have happened as a direct result of the fish emulsion. But from reading online, I wonder if this is an iron or nitrogen deficiency. How do I tell which one it is?

Thank you.