Sam, we have had a few guys report this issue...not sure exactly what in your tank might be causing it. The good news is that in all these cases, folks have told us that it stopped after a couple of days or even hours. At the very least, it means Nualgi is starting to work.I've noticed quite a few bubbles raising from my sand now a couple of days following dose two. Is this expected? Or could it be from something else?