I had a similar issue with a 45L I used the MGO, clay fert mix and capped with about 3 inches of sand. I should've soaked the dirt layer then capped to avoid the air pockets that were coming up for months. As long as there isn't a foul smell it should be harmless and just more of a nuisance. I have MTS snails that help aerate the substrate. It all eventually settle with no real issues but I don't have Co2 on this tank.