White sand getting Dark in fully cycled tank

[delphi]

I have a 55 gallon fully cycled fish only aquarium with 4 cichlid fishes, I have been running this tank for 6 months but for some time I am noticing that my white sand substrate is getting a bit dark. I does not look sparkling white that it used to look when I first set up the tank. I do regular maintenance of my aquarium and also siphon out all the waste particles but still no improvement. What can I do to keep my sand white?
I have attached the picture of my sand.

 

[PlantedCommunityTank132]

Either fish waste, or algae. If it's algae, you can simply woos around the substrate, turning the sand particles. Do you vacuum the sand?

 

[JustAFishServant]

Either fish waste, or algae. If it's algae, you can simply woos around the substrate, turning the sand particles. Do you vacuum the sand?

I agree with PlantedCommunityTank132 . It's most likely algae (or IMO, aufwuchs - a mixture of algae and small microorganisms). I've never experienced aufwuchs until recently and haven't gotten rid of it without a nerite snail, but I'm sure other snails and shrimp would do just fine as well

 

[Lily1]

I use the off-white playground sand from Lowe’s in my cichlid tanks. haven’t seen what you have there, the algae appears on the glass & hides. But if it is indeed algae, I’d vacuum it up and add freshly washed new sand.