Just to update you a bit , I did a huge water change today (about 40%) I think , cleaned the filter media with the water from the water change of course. Cleaned all the plants , bogwood, rocks etc etc . Scraped the glass with the magnet cleaner. Looks better now of course.
However if problem persists I was thinking to add another internal filter and change the filter media , filling it with ceramics and charcoal
Will keep you updated