Why are my plants rotting from the bottom up?

The fist pic is of one that just floated up.

I have bio active substrate caped with sand and I try to dose it with fertilizer ones a week but they still seem to be rotting.

Everywhere else I’ve looked says that it’s just plant melt from them being new or that they are lacking nutrients but I’m not sure which ones, and I’ve had them for a couple months now and they’re not growing back.