ive had this happen to me after about 8 months of dosing kalwasser to the tank everyday. There was build up on the vortech powerhead and was quite east to get off. Vinager bath, then i took warm rodi water let it soak in there, then it just fell off. Pump is good as new again
I had the same problem. It was back when I used a float switch to control the amount of limewater that was added to the tank. Unfortunetly, it was cause a decent amount at a time to be added. I believe this was my problem, it was adding too much at one time and to the sump where the pumps were. Causing high levels in this area.
I now add limewater by dripping, so it is added at a much lower rate and I haven't had a problem with the pumps since. This is going on a year or 2 now. HTH