Any Idea What This Is?

[Jeff]

Just a quick question, now that I'm really inspecting my two year old system more closely. Since my mass fish die off recently, I am overly paranoid and trying to rule out things as the culprit.

I've noticed that on my return PVC pipes (feeding water back into my tank), there is a crusty substance all over it:

[attachment=0:11a2tcg9]WeirdStuff.JPG[/attachment:11a2tcg9]

Any idea what that could be? It's that off-white color and you can see it on the 90 degree elbow. You can really see it around the bar code in the picture above. Is it just gunk? Is it water deposits from hard water? Is it some kind of calcification? More importantly, do you think it could poison the fish?

It seems to chip off easily if I run my fingernail over it.

I have never cleaned these return pipes (ever), so what you're seeing is just from two years of running the system. The water coming from these valves is what's being returned to the tank from the hydroton grow beds above.

Looking forward to your thoughts!