


How thick is the substrate? You may have stuff rotting in the gravel... or dead plant material? 5 is a big big crash. Something is decomposing in your tank in a major way.
Baking soda is a quick fix... not the answer. To dose your tank like that is the same as using a band-aid to fix a broken leg.
You may need to do some serious vacuuming... daily. Try doing it in quarters... and really get into that gravel. Make sure the substrate is no thicker than 1/2 inch... and have crushed coral mixed in.