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Regulating DIY CO2 level...

Hi all,

First post to this forum, so please go easy on me... ;-)

I've recently (6 weeks?) set up DIY CO2 into my 46G bowfront, but I'm still (3rd batch) having great trouble getting stable water conditions, and I think it is starting to affect the fish.

KH is 6.7 (120ppm) after the addition of baking soda NaHCO3 (originally around 2). This bit I can manage fine... Phosphate however is affecting this value - I have just under 5ppm.
So, the CO2 level is not easily calculate-able. This isn't really a problem - the lighting level isn't high so I don't mind less dissolved CO2 than that given by the normal tables. 

But the problem is the pH swing - this gets down to 6.0 or less over night, and up to 7 during the day, even with an airstone on (I tend to switch one on overnight to try and reduce the pH swing). Presumably this is because the phosphate is not buffering as carbonate would, so the swing is large? I think it is the swing affecting the fish, not the level of CO2 or pH itself - its certainly more than the 0.2/day recommended. Initially each batch produces loads of CO2 as well, so the swing amount varies too (I've added NaHCO3 to the mix to try and reduce this, but no luck yet).

So how on earth do I regulate the amount of CO2 coming out of the DIY bottle and keep the pH stable? And how do I calculate exactly what the actual CO2 level is? 

Any thoughts gratefully received...

Plants are doing great though!