Last night, I met something unexpected: a rising water level. You can see the level in the neck that it was at in the beginning and it was at the very lip with the Ramshorns hanging out there. I noticed them staying there and that's why I was thinking it wasn't getting enough oxygen exchange so I was going to lower the water level to create more water surface for oxygen exchange.
I have no explanation, either. The only possible thing I can think of is I put in a piece of boiled calcium that they use for birds in there as well as my other tanks. Nothing else could have made the water level rise.
I'm definitely not mistaken. It overspilled when I put the hose in to take out a little bit of the water. I'm going to be using a dry erase marker to see if the level rises again or not. It took a couple of days for it to fully rise, so we'll see in a couple of days, maybe...
Perhaps a temperature change causing the container to contract/water to expand? Maybe watch and see if the water level rises and falls depending on the time of day/room temperature.