gravel turning black

Question
I would like to know why is the gravel turning black in my
freshwater tank? it is 20 gal. and all babies(guppies).

Answer
Hi Cindy,

Are you vacuuming the gravel? It's necessary to do sometimes so that the mulm (definition here: http://freshaquarium.about.com/od/termsandtables/g/mulm.htm) that accumulates is removed from time to time. Most people recommend doing half your gravel bed once a month, then doing the other half the next month. That way you don't upset the beneficial bacteria in your tank.

A gravel vacuum looks like this:
http://www.pets-warehouse.com/pic-a/ALE11552.jpg
They start themselves, you just need to pump them up and down (they come with instructions).

If you mean it's turning dark on the front pane of glass, that's often times algae...you can move the gravel out of the way and scrape it off using a credit card or other similar hard plastic.

Take care,
Nicole