What is this???

[Taregreen]

I have three 10 gallon tanks I use for either hospital tanks or QT. I don't have anything in them, no plants, no fish, no substrate. Okay, there is a lava rock in one of them. I use a sponge filter in each. Can someone please tell me what that brown stuff is that collects at the bottom? I vac it out and fill with fresh water, but it comes back in about a day. What is it and is there a way to prevent it?

Thanks, 

Jay

[lefty o]

i would guess you have a lot of minerals, or sediment in your water supply. fill a gallon pitcher with water and let it sit for half the day, and see what settles out of it. beyond that, i dont have a good guess.

[KaitieG]

That's strange looking stuff.  My parents have water that grows iron bacteria (not high iron water--this is something different) and it looks kind of like that.  It usually takes a day or two to show up in non-agitated water (like, say, the unused guest bathroom).