Hi Ahmad,
Looks like melt which normally means too much light/for too long, therefore not enough CO2 with/or poor distribution
Start by reducing light to one tube if possible or removing reflectors, next move your drop checker around the tank to check CO2 distribution and also visually observe for dead spots, to improve flow upgrade filter/add a power head/ redirect spray bar.
remove damaged/decaying leaves so they don't reduce water quality further.
Start simple, you can hope it might be an easy fix!
Cheers,
Nutty