If you have had this unit running for a few years ,like you said, with few problems, until now and the filter on the front cover is clean , and yes it's the only one, and the unit is clean inside, as you said, then if the fan is still coming on, I would not be looking at the chiller as the problem. The problem is most probably flow. check the pump , hoses and even the chiller for calcium and algae build up (it is a reef tank), also how old is the pump ? just because it's not noisy doesn't mean it's still pumping the gallons per hour like when it was new. These Cl-650's run most efficiently at about 700 to 800 gph at the output, minus 100 gph for the chiller itself. also they work the best if the output is going back into the tank.
Also I got some better fuses from Pacific Coast Imports and I almost never blow a fuse and I am very under sized with a 650 with halides and over 300 gallons volume
Good luck with it.
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HAPPY REEF KEEPING !!______________________
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