Initial goal was to lower how much “stuff” I add to the RO water in an effort to supply the cleanest source water possible while still giving the plants what they need to grow. (started the decline sometime in December 2020).
I ended up “thinking” I was dosing my RO drum to 16ppm CaSO4 and 5ppm MgSO4 each time I did a water change. The method I was using to get to the 16 / 5 amount was to test the ppm TDS of the RO drum water after a fill up, then add enough CaSO4, MgSO4 and K2CO3 to get back to my target ppm TDS and 1dKH. (target ppm TDS value was based on dGH information supplied by Rotalabutterfly for Ca & Mg, then calculated out to an approximate ppm TDS which also included the ppm TDS of the actual RO water with nothing added).
Now, anybody out there figure out where I might have gone wrong yet?
It would seem 1dKH of K2CO3 actually adds a fair amount of ppm TDS to the equation. I was not accounting for that and basically the end result was my aquarium water column Ca & Mg levels decreased a specific amount with each water change. Kinda a reverse accumulation if you will.
I could not find any reliable info on how much ppm TDS 1dKH would add to a given volume of water so I ended up doing my own non-scientific experiment. Started with 1 gallon of TAP water and measured the ppm TDS of that. Then calculated how much K2CO3 would be needed to raise the KH of the gallon of water by 1dKH. Added that amount and gave it some time to dissolve (about an hour). Would seem that tiny amount of K2CO3 raised the RO water level 18.5ppm TDS.
My best guess is my aquarium water column was down to around 9 to 10ppm Ca. This, it seems, lead to many unhappy plants. Which lead to some poor growth. Which I believe gave the green dust algae a prime opportunity to say hello and stick around for awhile. For now, my water column will be correctly dosed to 20ppm Ca and a total of 7.35ppm Mg (additional Mg from MgNO3). Fingers crossed I will get back to happy plants again.
And if anyone is interested, water change volume is 75% per week and the latest “dosed” N/P/K is 12/6/18