Dirt stained the plaster ?

[Kevindong]

Here is the picture. I started to balance my own pool. But I have notice some dirt on the bottom of the pool. I usually just brush and suck it to the skimmer. But there is a area. I can't brush it up. Like stain.
Is that really common that we will always see the dirt on the bottom of the pool ? I just brush and vacuum yesterday. I have notice some dirt again. What cause it. How can I fix it ? I don't wann brush and vacuum the pool everyday though

 

[censored]

Two common tests to check your stain is a vitamin C tablet and a tri-chlor puck. There are direction under pool school on how to perform the test (nothing to it really, just hold it against the stain). If the vitamin C takes it away, it is usually a copper stain, if the tri-chlor takes it away it is usually organic. Your treatment will depend on whether it is metal or organic. Do the test and post results and the forum can help you from there.

 

[Kevindong]

Alright. Thanks