Hi Britta and welcome. Elisea is exactly right about the glass stones - they are smooth, which is ideal, and can allow the water to foul because of huge gaps in the stones, whihc is less than ideal.
Use a turkey baster to suck up what you see trapped in there on a daily basis. And when I use glass stones, I remove everything from the tank/bowl and wash the stones completely in hot water . NO SOAP. Then pour them back in, add water and dechlor and whatever other decrations, and finally the betta (make sure the water is warm, not hot or cold).