White line on bottom of tank

[zippy]

I have a white powdery line on the bottom of my tank looks like powder directly under the lights. I can wipe it away with my glass cleaner but it comes back in a day or too. I don't believe it is some kind of algae.
Any ideas
Zippy

[Filip]

I have a very hard and alkaline tap water with 330 TDS and 16 Dkh (280 )KH hardness .
I use to get this powdery carbonate deposits on my bottom glass after a big and unaged WC .
Now I age half of the water prior WCs and this powdery calcium carbonate deposits is gone .
Maybe that's what you see in your tank too ?

[zippy]

Thank you for your suggestion. It could be I'll check it out.
Zippy