I was going to mention to go see your tank, lol!
My degassed pH this morning, after CO2 has been off for 14 hours, is 7.2, which I believe correlates to 7.5ppm of CO2. If my goal is 30 ppm CO2 during the day, then my pH should be about 6.6...
It's generally accepted now that those kh/pH tables for co2 concentrations have flaws in them so it's best not to focus on absolute numbers. Too many assumptions about unknowns. Especially with regard to starting co2 concentrations. Which is why you see people on here dropping pH by 1.5 and being successful.
In my tank it takes somewhere around 36-48 hours to fully degass. But I have fairly low flow. I don't worry about an air stone at night. I just know that my degassed pH is about 7.1 so I target 5.9/6.0ph during lights on for a drop of roughly 1.2, starting before lights start to ramp up and ending when the lights finish ramping down. And in my tank that seems to work fairly well. I had started with only a 0.5 pH drop but didn't like the results.
So with your tank, and a kh of 4 right now, I would say your degassed pH is probably higher than 7.2. And you are probably correct in saying that it's more like 7.6ph. A daytime pH of 6.6 would be real close to that 1.0ph drop. Depends on what your actual degassed pH is. If you can, it might be worthwhile to see if you can drop that even lower to a 1.2 drop, maybe even 1.5, or there abouts and see what happens. You could go even lower as the plants won't mind. The Amanos probably would though.
Speaking of which (I didn't catch your post), 70 degrees is at Amanos low end. Not something you would want to stay at. By itself it wouldn't do much but maybe in conjunction with everything else it might speed things up. Algae wise. Even 72 would help. My tank is right around 73 these days. It does get warmer in the summer months, which here in my part of California is any day now.
On a side note, I had my son clean out his 40 breeder last night and catch 2 fish that had been in there since I bought it off Craigslist awhile back. We are concentrating on raising shrimp in this tank. I forgot that they were Celestial Pearl Danios. When I got the tank it had a kh of 0.5 and a gh of 3. So yeah, Celestial Pearl Danios can handle a wide range of water parameters, lol!
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