It might be that your water tap water is 6.9 or your substrate just keeps it there. The charts just measure dissolved solids. They assume that all your dissolved solids are Ca and mg. There are many other dissolved solids that could be in your water. SO you can't use the charts.
You know two things:
1. your pH stays at 6.9
2. your plants use CO2 and aren't getting enough as they are suffering.
You need to use a drop checker. It will be able to tell you if you have 20-30 ppm of CO2 in your water (you fill it with 4dkh water). Then you can set your CO2 to maintain the drop checker to a green color by setting your ph controller lower and lower until it turns green. That is a starting place. From there you can continue to increase the CO2 until your fish tell you it's enough. Do not increase past green unless you are home to monitor your fish.
My pH monitor is set at 5.8. It give me a yellow to clear color in my drop checker and the plants are thriving. All my fish are fine. I have 3 varieties spawning in my tank. I do regular 50% water changes on Sunday. My CO2 comes on every day for a little while.
You know two things:
1. your pH stays at 6.9
2. your plants use CO2 and aren't getting enough as they are suffering.
You need to use a drop checker. It will be able to tell you if you have 20-30 ppm of CO2 in your water (you fill it with 4dkh water). Then you can set your CO2 to maintain the drop checker to a green color by setting your ph controller lower and lower until it turns green. That is a starting place. From there you can continue to increase the CO2 until your fish tell you it's enough. Do not increase past green unless you are home to monitor your fish.
My pH monitor is set at 5.8. It give me a yellow to clear color in my drop checker and the plants are thriving. All my fish are fine. I have 3 varieties spawning in my tank. I do regular 50% water changes on Sunday. My CO2 comes on every day for a little while.
