Brown Marks

[Nottm_George]

After returning from holiday we found that there were dark brown marks on some of the rocks and on the gravel we have emptied the tank and washed the gravel but it as returned. Any help please.

[Tilly2]

Are you talking about an aquarium, Nottm?

[Buenchico]

See if this helps:
http://freshaquarium.about.com/cs/maintenance1/p/algaebrown.htm

[B00]

If this is indeed an established fish tank (fish in it) I really hope you've kept some of the old tank water in it before refilling it again, if not you're very much in danger of having some very dead inhabitants.

[jomifl]

Too much light or too many nutrients if it is as Buenchico's link suggests. Do you have aquarium lighting?

[jomifl]

When replacing aquarium water with tap water, leave it in an open container for a day or so especially in the winter. Chlorine in is not very soluble in water and will escape to the atmosphere.