I'd run a blackout on this tank (or pull it all out and start over lol).
To blackout the tank:
Clean tank and filter really well, manually removing as much algae from leaves as possible. Remove any dying leaves.
Do not dose ferts or carbon.
Do not feed fish.
Cover the tank so that zero light can get in.
Leave the tank covered for 4 days - no peeking
Uncover and do a large water change and clean filter as it may have become clogged with dead algae.
Resume dosing schedule.
Only run your lights for 4-5 hours a day for a while afterwards, slowly increasing over several weeks.
You may lose some leaves on your plants that are in really bad shape but it'll be worth it IME.
You can repeat the process as often as needed while giving time in between to let your plants/ fish bounce back.
As for what caused this it looks like too much light and not enough flow to me, possibly a lack of good tank maintenance as well
aaaand this is an old thread *doh!*