Hello From Florida's Space Coast!

[jessithebuckeye]

Yes, that is food at the bottom. Their food will sometimes sink to the bottom and they like to pick at it as if it is algae. I really have to be careful not to over feed them!

 

[daizeUK]

I'd suggest drastically cutting back the amount of food you give.   The food will rot and foul the water quite quickly if left in the tank. 
 
I only feed my fish as much as they can eat immediately - if any sinks they should be hungry enough to go hunting and clear it all up within a couple of minutes. 

 

[jessithebuckeye]

Will do. Thanks for the tip.