At which points within your chlorination cycle are you measuring your pH and are comparing measurements with each other? We say that adding bleach has no net effect on pH. That means that chlorinating with bleach is pH-neutral over the whole chlorination cycle. Adding bleach will increase pH, the following usage of chlorine (by UV-decay and FC killing bacteria and algae and oxidizing stuff) will reduce the pH again more or less to where you started (ignoring other effects on pH). If you are now comparing a pH-measurement that was done just before a bleach addition with a measurement that was done just after a bleach addition, you might see a change in pH that is not actually a real pH-drift, that value will come down again once the chlorine has been used up.
Here an example calculation (using chem geeks poolequations spreadsheet, which is pretty accurate) with TA=50 and CYA=60 (as you have posted in your other thread). Let's say you are at the min FC of 5ppm for your CYA and pH is 7.7. You now add enough bleach to bring your chlorine to the upper end of your target range (FC=9ppm). This will increase your pH temporarily to 8.11. The following usage of 4ppm worth of chlorine (bringing FC down to where you started) will reduce pH back down to 7.71 - the whole cycle is more or less pH-neutral - of course, other effects that have happened in the meantime like CO2-outgassing will result in not getting back to where you started.
When working out your pH-drift due to CO2-outgassing, it is important to compare pH-measurements that have been done at the the same point within your chlorination cycle. If you are comparing pH-measurements at the bottom of your FC-range with pH-measurements at the top of the FC-range, you will see fluctuations that are not representative for the actual pH-drift.
That is the only explanation that I can come up with. I don't think that a pH-drift of 0.4 per day can be explained by aeration, esp. not with a TA of 50ppm. My pool has pH-drift that is usually below 0.04 per day.
Work out your pH-drift based on measurements from the same points within the chlorination cycle, and add less chlorine more often to minimize the fluctuations within the cycle. These fluctuations are more dominant at lower TA: At TA=80, the same addition of bleach as above will bring pH only up to 7.97. Low TA is good to reduce the drift due to CO2-outgassing, but you have to be aware that the fluctuations within the cycle will be higher. That's why it is not recommended to go below a TA of 50ppm.
