Unknown Bubbles

I have recently had bubbles that just won't go away they have been in my aquarium now for about 2 weeks and through two water changes and they keep coming back. I don't know what to do about it? I have a bak pak skimmer hanging off the back letting in a few bubbles but no more then normal. The bubbles seem to be coming from the sand I am not sure how they are getting there but they keep accumlating there and then releasing up to the top of the aquarium. I have attached some pics of the bubbles in the sand and on some of the rocks not sure what I can do to fix this any ideas?

Thanks,
Scott

 

Part of the normal process. Its nitrates being broken down into CO2 gas More common in younger tanks.

 

How long has the tank been set up. This is usually part of new tank syndrome. During the first year of a tank the sand will produce and release bubbles. There's not much you can do it's just a normal part of a tank maturing.

 

The tank is about 3 years old but had a recent crash so I am not sure if that would be considered having to restart and deal with new tank syndrome again?

 

How long does this normally last for if that is the case?