How much light is it getting? It's been my experience that they require quite a bit of light. I keep mine in an east-facing window (quite a bit of bright light here in TX) in a fishbowl covered by a clear glass plate. It is completely filling the bowl. I would let the plant fill in a bit and then remove the unsightly fronds.

Ferns do not need any fertilizer, ever, and are more often than not damaged by it, so you shouldn't do that anymore. Are you using distilled, RO, or rainwater exclusively? You should be. Is the soil perhaps too wet? I would doubt this to be the problem, since ferns can handle an awful lot of water, but it could happen. It sounds like some version of damping off. You could try leaving your terr open a bit for a few days and see if that helps. Keep an eye on it though, because the decreased humidity could lead to the fronds drying up, and you wouldn't want that!

Thanks for the info. I only use purified water that you can buy in the jugs. When I can I use rainwater and sometimes melt snow and store it. I thought perhaps it was damping off- but it doesn't occur at soil level. It occurs 3 inches up and they look perfectly healthy until they reach a certain height. Then at that single point it rotsbrown, the bottom turns black (but the stems turn black anyway) and then collapses. I don't fertilize often,maybe lightly once a year during the spring. I did read in a fern book that you can use fish bone to restore nutrients to plants in terrariums and fertilize at half dose every 2 months. Maybe it is an overwatering issue because it is winter. Thanks!

Who knows? I find that my Adiantum does this sometimes, too. It is currently in the process of re-growing all of the fronds that died recently. This is the third time it has done this since I got it. As long as it keeps getting new fronds, I wouldn't worry too much. I do think that mine was staying too moist though. And it was getting algae on the leaves because it was right next to my waterfall.

Ferns like fertiliser. I regularly give my ferns seaweed and fish emulsions.

Hi
I have the same experience with mine. Starts off doing great then will start to brown,New fronds are weak and undeveloped. I move it to the outdoor terrarium and it's growing like gangbusters lol. Haven't been able to figure out the difference between the two locations particullarly in winter. Of course the indoor one has artificial light whlle the outdoor one has only natural light. But there are 3 layers of epi's above it not to mention the lattice on the roof and is completely shaded from 10 on during winter.
To my mind it should be doing just the opposite.lol
I also grow maidenhairs in tree-fern fiber along with Cattleya orchids suspended from the roof. No matter what I do they will go dormant during winter.No matter what temps or humidity I maintain. Come spring they start growing again.lol.
I was so sure that they would grow well in the indoor terrarium where everything is so controlled.
Gary