It's Anacharis not Anubias.
Trim off the sickly part and replant. Are you adding any ferts ?
Do you have any fish that might be eating the plants ?
There at least three species of plants sold as Anacharis. One of them cannot be kept in warm water and will die off. So if you keep losing your Anacharis, you may need different plants.
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Oops I meant anacharis. I only have Angels in the tank, I'm not adding any ferts and at my lfs none of them were doing this so maybe the ones I have are the cold water species?
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