White residue... what is it?

[Jonathan Kadish NYC AA Chair]

I have noticed this white residue on some of my expanded clay rocks and I am wondering if anyone knows what it is... wasn't there before I started cycling. I did have some mildew smells early on but i put a small fan on the system and that went away. I have seen other people on Youtube with a similar residue which they didn't know either.

[Rob Torcellini]

Do you have any salt in your system?

[Vlad Jovanovic]

I don't know what that is in your picture, but I have noticed 3 different similar looking 'white stuff' phenomena caused by 3 different things in my situation. 

1 - Some of my hydroton just seems to come that way, probably some kind of fire-proof material in the kiln?

2 - When I use my tap water (which comes from a local aquifer) in my little hydro-rigs, the hydroton will relatively quickly (usually hydroton at the 'water line') develop white residue which I just pass of as mineral precipitates. (This wont happen when I've used distilled water)

3 - One time on one basil plant it was mold in (just that one net pot). But this was slightly fuzzy sort of white when looked at through a magnifying glass and not chalky mineral type deposits. 

Sorry if this isn't much help in identifying whats in your pic...

[Jonathan Kadish NYC AA Chair]

No salt, it is NYC tap water.

[TCLynx]

tap water tends to have minerals in it.  Like calcium carbonate, think lime scale or hard water stains.  That is what I would guess the white stuff is.

[FormulaX]

That's just mineral build-up, I get that with Philly water as well.  It's not visually pleasing, but it's fine.

[Jonathan Kadish NYC AA Chair]

Well, more nutrients for the plants then :)