It could be a water quality problem if you're using tap water. The problem with a lot of tap water is it's loaded up with all sorts of chemicals which epiphytes sitting up in trees in the rain never come in contact with.

I have used tap water on occasion. I try and have water sit out overnight as much as possible, to evaporate any chlorine.

I place mine in the sink and spray it down all of the time with the sink sprayer. When i have it outdoors, I hose it down with my garden hose. They don't care if you use tap water. I would try and increase air circulation after you water it. I let my ceiling fan run to help air it out afterwards.

I will try that as well thank you! I'm not quite sure why only that leaf is yellowing-- and it hasn't spread down that leaf either, it's just in the one spot.

Also, as for the pups, there's no need to separate the plants. It's their nature to grow like this. Perhaps when your fern gets to be much larger, then you can divide it. But if you tried to divide it/them every time they did this, then you would be doing this all the time. They will grow perfectly fine being clustered together like that. That's just how they grow.

Photo Synthesis, some tap water will end up killing them, it does depend on the quality.

I guess it can depend on where you live. Here in Arkansas, we have no shortage of water. Being the seventh rainiest the state and all. I've looked at my local water municipality's website, and our water isn't heavily fluoridated or chlorinated. Just the bare minimum that's legally required to treat it, I guess. But I've been using it for years on my staghorn ferns with no harm done to them. Other plants of mine still don't care for our tap water. For them, I use the rainwater that I collect and store throughout the year. Unless they're getting rained on directly, I always spray them down with the hose.

That being said, she should still increase air circulation after watering to help dry it out. This should be done for any epiphytes.

I have the plant a good 5 feet away from a South facing window, so it gets really good indirect light all day. We have pretty good tap water here (although the city has been working on the pipes in our neighborhood for the last couple of weeks, so it maybe that they've done something to the water to "clean" the water when they're done). To offset this I've been airing out the water each night before watering the staghorn fern. So either the water isn't getting distilled enough, or it's the air circulation.

@Photo Synthesis, thanks for info on pups. I haven't been able to find much information on when it's a good time to separate them, just how to. It's good to know that them being squashed like that is OK.

I believe they prefer tap water as opposed to distilled or spring water.

UPDATE: the sick leaf fell off, and all the rest of the new growth is growing well. So it doesn't look like whatever was wrong with the one leaf has spread.

New question: Since the fern /is/ growing, should I fertilizer now? Or wait till spring? I'm always super cautious fertilizing newly moved plants, and doubly more so with the fern.

Hi, The yellowing leaf is because of lack of humidity, I had this problem and moved to a bathroom and it's never looked better!

I've had staghorns for 45 years and never fertilized once. If I was keeping one indoors I would mist the base until it was dripping once a week (I live in the Texas Hill Country northwest of San Antonio). Mine are on the outside of hanging wire baskets outdoors. I carve the base of the plant so that it barely fits through the holes in the basket. Then I stick the plant into the basket from the outside. Finally I fill the wire basket with sphagnum moss to hold the plants in place. If it doesn't rain for month or so I will water it with the hose. The plant really likes being saturated by a week of rainstorms and then left alone for several weeks.

Sadly our bathroom is completely cave-like, with no windows. Otherwise Staggy would have been in there from the get go!

I will try and water/mist even more frequently than I already am. So far I haven't had any repeat instances of yellowing leaves, probably because I've been drenching the fern for longer.

A once a week soaking sounds like a lot of watering for the time of year, does it feel dry once a week?

It does, yeah. It gets maybe into the mid 50s on a really cold day, and 30's at night if we have a freeze. Average nighttime temp is mid 40s.

I keep it well away from any air vents, so no direct heat is blowing on it. And I always check and make sure it's dry before I water.