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I have my crypts planted in gravel. I feed them regularly with root tabs and Flourish Comp. They never did much dying back, though. Crypts are sensitive to being transplanted, but I only lost a leaf or two off mine. And that was some time ago. They've stayed nice and green for the most part.
They must like the environment they're in.
Yours should come back, especially if you feed them.
20 gal. high: planted; 5 white cloud minnows, 4 golden White Clouds, several RCS, 2 blue shrimp, 5 Amano shrimp, several snails; Azoo air. 65 gal: planted; 6 rosy barbs, 6 yellow glofish, 3 red glofish, 3 zebra danios, 5 white cloud minnows, 3 dojo loaches, 6 crimson spot rainbow fish, 12 large Amano Shrimp, several snails; AC110.