Originally Posted by nickdu
It seems like i have trapped gas in the substrate because bubbles are big. There are small ones as well though. But how does gas appear in substrate to begin with? It should still be photosynthesis because i only get big (& small) bubbles near plants. My entire tank isnt planted yet. Hoping hairgrass will carpet.
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.