No, used that for years. I might have cleaned the float valve once a year, that was it.

Not the doser unit itself, but a 5g jug tied into the float valve

I have been running my kalk in my top off water for years with no problem. I only recently changed to a reactor because kalk tends to eat up maxi jet pump impellers quite frequently. The key to stability is top up slightly less than your evap rate then every couple days manually top up with rodi just to get your water level back to normal.

Where do you get kalk from? I know the fish stores sell it but it's fairly expensive.

Kalkwasser or Kalk powder can be purchased in most aquarium stores...

See if you can find someone that ships pickling lime from the US. Also, one of the members here used to buy tubs of it in Vancouver somewhere and sold it for $5 for a grocery bag full. Not sure what it was labeled as though.

Here in town;
- Eli sells it bulk
- Pickling lime at Superstore or Canadian Tire with the canning section but you have to be fast (usual months are June-August for canning supplies being big and out on display).
- Sometimes its at Co-Op in the spice and seasonings aisle.