is it rust or???

[louise]

I do a gravel clean once every 2wks max and i have noticed there is like rust on the gravel!? what is it and where is it from? All of my tests are ok?

Sorry for being dumb but i need to know if its good/bad or what, thankies

 

[MAM]

could you describe it a bit more?? could be waste or algae

 

[louise]

urm i have no idea. My cat fish loves sucking on it but its never been there before, it looks like red algae, but whats it from?