Although plants aren't my strong suit, I can try to help out here.

How much light are your plants getting? They may not be getting enough. Especially if they are all dying, that means they're not getting enough of something (be that light or nutrients of some sort). Your pH is also slightly high according to the information I've found on aquatic plants (in general). I linked the one article I am getting the specifics for aquatic plants (below).

How often do you change your water, and how much during each change? Do you have a lot of surface disturbance? That would oxygenate the water, which is great for fish and not so good for plants.
What I've reference in the post

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Phishie
Site Coordinator

"An ounce of prevention is worth a pound of cure."