If you used SW
to clean it then it shouldn't have harmed the bacteria really. But that's the only thing I can think of other than something dying and rotting in there. If your snails don't normally sit out of the water like that then it's probably a good indication that something is wrong.
I'm surprised the fish look unaffected.
Have you tested for nitrites lately? If the bacterial population has indeed been wiped out then the presence of ammonia will cause it to rebuild, but unfortunately that will mean ammonia gets converted to nitrite which may well be deadly in such a small tank.
You might want to consider relocating the fish temporarily and then letting your tank cycle.