Rust on rocks

[Woody]

My 46-gallon freshwater tank contains a structure of caves that I build with large, flat rocks that I collected near my home. Several have sections of the rocks have turned a rusty orange color and I think they may contain some iron. This doesn't seem to bother the fish, and it's certainly not bad enough to affect water quality in any visible way. Should I be concerned about long term effects?

The tank contains corydoras, a pleco, a couple of gourami and a mix of bala and apollo sharks.

 

[mike777]

Are you sure it isn't algae?

 

[Mako]

Yeah, the rocks that i collected did the exact same thing. It also spread to my gravel, and eventually the glass. Mine was due to algae and I now just scrub the rocks and clean the glass with a scraper when it gets really bad. I don't think that rocks would contain enough metallic iron to cause the amount of rust you are describing.
It sounds as though the 'rust' is quite localised on the rocks, mine did that too, don't ask me why!
Hope this helps!

 

[Bol]

Is it somewhat easy to wipe/scrape most of the "rust" off?

If so, what you have is probably what's called "diatom algae", which is not really algae. Shouldn't hurt your fish, unless it goes unchecked.

 

[Woody]

I hadn't though of algae, mainly because the orange stuff appeared within three or four days of placing the rocks in the tank, and it's not spreading. It appears only on certain sections of certain rocks. Between the fish appearing healthy and everyone's reactions here (so far), I'm not going to worry about it. It actually looks rather nice.