strange blue spot in sand

[Bambithegirl]

Hi all,

This is the second time ive noticed a small blue spot in my sand (pool filter) in 7 gallon tank. I only have one betta in there and coincidentally it is the same colour blue as his fins.

Any ideas?

 

[Delapool]

Can you include a photo?

 

[Bambithegirl]

Unfortunately I didn't get a pic and I have since done a water change but its happened twice so I will be sure to take a pic when I see it again