Bubbling is completely normal as monster advised. If you see 'mounding' of the substrate give it a poke to allow the gas pocket to release. Say the substrate is flat and then you notice a little hill developing it's simply a build up of trapped gasses and nothing to worry about.
Even if you notice different odors directly over the tank while releasing gas bubble by poking the sub the tanked critters will be fine. The gas bubbles travel through the water rapidly to pop at the surface. It's all part of the organic decay process.
Welcome to dirty tanking