Unable to remove gray film on aquarium rocks and decor

[Chili_girl]

I have been unable to remove gray film on sandstone rocks in my Mbuna tanks. I understand porous rocks are not ideal but they were sourced from my property and I love the color. I also have some Texas holey rock other decor that also have this gray film. Not Black Beard algae, this is different. 

 

I can't scrub it off with a stiff brush and water. Tried soaking them in bleach water but only turned them white. Film still won't come off. I could start over with a new batch of rock but these have natural holes, caves crevices and neat depressions nice for Mbuna so I'd like to keep them if I can figure out how to clean them. Any suggestions?

[xXInkedPhoenixX]

I would try hydrogen peroxide? Can you post a picture of the rocks both in and out of the water? Maybe some that haven't been in your tank so some of us can take a look and determine what it might be.

In the meantime you can take a dropper and put a little hydro on there see if that does anything.

[Chili_girl]

 

 

Here's an example of the sandstone rocks before and after in the tank for a few years (dry). All the rocks and decor and covered with this grey stuff.

Peroxide didn't take it off.

 

[Chili_girl]

 

 

 

Here's another picture of them wet.....

 

[Tony s]

From the look of it. The top gray rock is actually the real rock. The brown looks like an oxidative coating. Probably high in iron. That’s just dissolved over time. If you have a smaller brown one that you don’t mind sacrificing I’d take a hammer and split it to see what’s inside.