Hi Tanja .
Very low tap water KH of less than 4 degree could very well leed to this fast ph drop in your tank due to decomposing organics creating acids, and not in your barrel where the water is clean .
You should start raising your KH with commercial KH buffers or crushed coral in the filters , as many members here use to keep their KH and PH from crushing.
The safest way for KH buffer is to experiment and measure in your aging barrel rather that directly in your tank.
And with crushed coral , you can put it straight in to the main tank filter , because its impact on KH raising is not sudden but rather gradual.
Please share some photos and info about your tank too , because here at " Simply " we "The discus geeks " love to look photos of discus and different discus tanks