I just changed the lighting over my 75 gallon tank to 2-250w metal halides. they are 10K single ended not sure of who made them, before just 4 VHOs. It was a fish only for over a year, I traded most of the fish for small coral.

I did a lot of reading and started the halides for a few hours a day slowly raising to 8 hours over a few weeks, but now all of my sandbed and rocks are full of bubbles, a lot of them. Looks like a pan starting to boil. The sandbed is not deep only an inch or two in most places. What's going on?? When the halides are off they go away but come back after only a few hours the next day.

PS no sulfer smell when they reach the top.