Sand becoming discolored

[jcmcn5]

I just began stocking my 40 gallon two days ago. It is fully cycled with one half of the bottom a sand substrate, and the other half red flourite. After adding 8 fish to the tank on Friday, I'm noticing that a full inch of the surface of the sand from the glass has become a slight shade or rust/reddish/brownish color.

Any idea what might be causing that? Could it be dissolved but uneaten food? the tropical flakes I used are red but I didn't use that much and even it I did, why is it only the inch of the surface next to the glass that has become discolored.

Any thoughts would be appreciated.

 

[Rivercats]

A picture would help alot but my guess is you have diatoms. One way to tell is do they wipe off very easily? Diatoms are self limiting and will eventually go away on their own once they use all the excess silicates in the water. Limiting you lighting to 6 hours will help the amount of diatoms that grow. One question tho is your tank planted? If not you can use a phosphate remover in your filter (PhosGuard is a good one but there are others) as phosphate removers also remove excess silicates in water.