Try using a coarse algae pad and some non-iodized salt (many say iodized is OK, but I'm from the old school) or if that doesn't do it, use some vinegar (clear), scrub well and rinse well.
If they don't work, chuck that puppy and buy a new panel.
You could pick up an all-purpose fish food like Kents and dose at 1/2 strength. See how that works out and increase the dosage if needed. IMO, there's no sense in getting 40,000 elements do dose individually when an all-purpose will probably do the job in your tank.
Don't forget your water change weekly. It will also add back some of the essential Traces for you as well as keep the water clean.