What causes these high iron deposits in your water tank and are they harmful?

The most common sources of iron and manganese in ground water are naturally occurring.  

The problem is when these "naturally occurring" iron deposits are flowing through your water pipes they can catch and stick which can affect water pressure and eventually become clogged and damage your pipes and appliances. 

You are looking at an 1800 imp. gal ribbed poly (below ground) water tank with two points of entry.  The two openings make it ideal for cleaning.