We've got a set of FAFCO solar water heater panels on our roof that came with the pool when we bought the house a little over a year ago. At that time, we were getting good flow through the system, including the solar panels. Over the last year, we've noticed that the flow through the solar panel has diminished (judging by the amount of water that spills over the waterfall from the spa to the pool). With the solar panels off, the flow is good, and my DE filter shows about a 1-2 psi increase over what it was when I last backwashed. The solar panels are now increasing the filter pressure by about 8psi. Our area has high CH in the water (last measured at 500ppm... tap water is high, so we can't fix this by draining/refilling). I suspected that we might have calcium buildup in the panels. I tried to force the water through the panels backwards and let it drain out from the solar panel inlet, hoping to dislodge any debris from the panels, but this didn't help. Any ideas on what I can do next? Are there any chemicals that can dissolve the calcium in the solar panels, without destroying or deteriorating the panels themselves?

Thank you.