Over the winter, these small brown dots have been appearing on some of my birds nest fern's leaves. The fern is planted in Al's 5-1-1 mix and receives filtered light from a southern window (blinds barely cracked). I opened them a bit more to take my photos.

It is possible that I've overfertilized the fern over the winter, because I've been watering with the same solution I use on the gritty mix plants. (weak foliage pro) I recently realized that was probably a bad idea for this plant.

I have not found any evidence of scale on the underside of the leaves nor anything else I can find as a common issue with these plants. The spots don't seem to be concentrated near the bottom or center of the affected leaves, and I turn the plant regularly so I don't think it's a direct result of light/draft/etc.

Ideas?

Thanks!

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