sciencefiction
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The parameters of the tank water before the change were: Temp 25.0°C, TDS 139, pH 6.5, NO₂⁻ 0, NO₃⁻ 40, KH 3°, GH 5°, O₂ 2mg/l
The parameters of the tank water after the change were: Temp 24.7°C, TDS 121, pH 7.0, NO₂⁻ 0, NO₃⁻ ~30, KH 3°, GH 6°
What is the Kh of straight tap water? In order to get a KH 3° in mixed rain/tap water in proportion 9:3, , your tap water must have a Kh 12°, That's nearly double the Kh of my hard tap water here and it looks unlikely to me your tap water has that high of a KH. MgS04 doesn't alter the KH. Unless there's something in the rain water that adds to the KH... which is again unlikely if its pure rain water.
Something doesn't look right to me in those tests and final results. I'd switch the CO2 off for a day next time to get a non-co2 PH of the tank water. Also, you may not be really matching the conductivity/TDS of the new water to the tank water because you are adding MgS04 to the mixture. I know you said it only raises the Gh by 0.5° but I'd be curious to know by how much it raises the TDS of the new water mixture? What's the TDS before and after you add the 10ml MgS04 to that12 litres of mixed water?
Have you tried just ignoring any KH, GH measures, forget about the MgSO4 to match Gh, etc......Don't use any tests bar the TDS meter and mix tap and rain water to get your desired TDS, e.g. 120ppm if that's what you want to be the target long term TDS. This may take a different proportion of tap/rain water, depending on your tap water. For example, my tap water has a TDS of around 250ppm straight out of the tap. To get a TDS of 120ppm I'd need a mixture of 50:50 tap/rain water or just slightly lower on the tap part.
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