Well done.
I encourage rainfall capture and my signature has a link to some info.I come to this subject as a Civil Engineer who deals with water, not a Chemistry bloke.
Your test results are a bit odd.
pH is 6-0 but you have alkalinity of 40 ppm. Alkalinity is part of the pH area.
From;
total-alkalinity-role-water-chemistry
"Total alkalinity is a measurement of the concentration of all alkaline substances dissolved in the water that can both attract and release Hydrogen ions (H+). This interference with Hydrogen is why alkalinity buffers against change in pH. Total alkalinity is primarily bicarbonate, carbonate, and hydroxide, along with a few others like cyanurate alkalinity.
When acid is added, these alkali have the ability to neutralize some of the acid. In simpler terms, total alkalinity is a measurement of the water’s ability to resist a reduction in pH.
From;
/ph-and-water
Some info pH is a measure of how acidic/basic water is.
The range goes from 0 - 14, with 7 being neutral. pHs of less than 7 indicate acidity, whereas a pH of greater than 7 indicates a base.
pH is really a measure of the relative amount of free hydrogen and hydroxyl ions in the water.
The colour is from something that is in solution, it cannot be filtered out.
- What type of roof material are you catching the rain from?
- Would leaves be staining the water?
- What amount of water do you use per day?
The larger the storage tank, [ say 5000 gal ] the more time water has to clean itself. I wonder if you are using water too fast for nature to help?