glass tops hardwater stains

CLR (calcium and lime remover) is probably one of the best options, or you can try scraping with a razor blade if the buildup is really heavy.
 
any grocery store or hardware store
 
Is there any way to PREVENT the white stains. Or algae buildup on the top glass?
Not to hi-jack, might be relevant to the OP. But I noticed my staining has increased since adding a bubble-disk, and algae growth right above it has increased approximately 847.3% (number may not be entirely accurate...)
 
I second the recommendation for CLR. It works great and has other household uses. People run it through coffee makers to clear out deposits, for example.
 
Yes, just rinse the glass thoroughly after you clean it. If it was toxic you wouldn't use it in a coffee pot...
 
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