What type of substrate are you using? Often times the matter in your substrate will break down and dissolve into the water column. As a byproduct of the decomposition there is gas that sometimes will get trapped in your substrate if the substrate is very compact (I.e. sand). Over time the gas bubbles get so big that they force themselves up to the surface often times around your roots since that's an easy way to escape. Gas is common with soil substrates as these carry a lot of organic compounds. If you place your hand in the tank and press on your substrate you'll sometimes see that bubbles come up - that's the gas that's trapped in your substrate. Nothing to worry about if this is the case - it's indicative that your soil is doing what it's designed to do.
On the flip side it can also be something bad. If you smell rotten eggs when the bubbles come up, chances are there's some anaerobic bacteria that's feeding on your soil/detritus in the substrate. If this is the case it often indicates that there's low 02 content and you'll need to aerate the substrate (you might want to take the fish out first). This is common with densely rooted plants. We'll need more information from you.
I'm using a mixture of fluval stratum substrate and eco-complete substrate mixed with black gravel. There is no sand like substrates that can trap gasses it seems. the pieces are all a couple mm thick so the bubbles can escape if they need to. I'm not sure what you mean by densely rooted plants, most of my base is not covered in plants but i am hoping to get it densely covered in hairgrass. The parts that do have hairgrass... some spots are dense it appears but other parts do not look dense and they still have the big & small bubbles flowing up. I haven't smelled the bubbles but from the looks of the hairgrass, they appear to be doing better than they were. Not 100% sure yet though... need time to see difference.