Get a pH meter and watch.
It's just like the CO2 chart. It's "Holy Dogma" wisdom that you can reliably determine the CO2 concentration by measuring the KH and pH. The mathematics of exponential functions say otherwise. But the mathematics are ignored because "The Chart" is the "Holy Dogma." Fact is that it is virtually impossible to measure the KH (and in most cases the pH) accurately enough with "match the color" test strips (or 1 drop = 1 degree KH titrators) to get good enough numbers to plug into the formula and get meaningful results. If one reads the discussion on the Krib, the guy that posted "The Chart" over there admits this.
On turning off CO2 at night...
Think about this. If you leave a soda bottle open for 12 hours, it goes flat. The ratio of surface area to volume is a tiny fraction of that in an aquarium. Yes there is a much higher initial concentration of CO2 in soda and it looses CO2 much faster in the beginning because of that. I'm just trying to invoke analytical thinking.
If you turn off the CO2 in an aquarium for 12 hours, the CO2 concentration is going to *approach* that of the air at the end of that period. Don't take my word for it. Try it yourself (with a good CO2 test kit... not "The Chart".)
So figure if you turn off the CO2 at night, you're going to loose a very significant amount of CO2 by 12 hours later. Don't believe me. Test it with a CO2 test kit and see for yourself.
If you want to turn off the CO2 at night -
What either works or doesn't... kills fish or doesn't.... is the difference between what the pH is when the CO2 is on and what the natural buffers in the water "want" the pH to be (i.e. what the pH slowly returns to when you turn off the CO2).
In my opinion, this is the critical information Florida Driftwood, Yamato, et al probably know but fail to state clearly.
Draw the pH down by .5 with CO2 and you can very safely turn off the CO2 at night. Draw the pH down by "lots" more with CO2 and you're asking for trouble if you turn off the CO2 at night. Many will debate what constitutes "lots."
I would not subject my fish to an everyday pH fluctuation greater than .5 or so (remembering that pH is a logarithmic scale and a change in pH of +/- 1 changes the acid/alkaline ion balance by a factor of 1000%.) But that's just 1 person's opinion. If we can agree that a "big enough" pH change is, at minimum, stressful to fish... Then why in Heaven's name would one want to "push it to the limit" every day? It would seem prudent to try to minimize all sources of stress to both fish and plants. Seems like common sense.
Now if one claims that pH changes of any size are not stressful to fish, then I suppose that's a Tetra of a different color.
On the other topic...
I thought I remembered you saying in another post that some of your fish got Ich during or after the CO2 spike/pH swing period. If I'm mistaken, chalk it up to early senility.
No, I wasn't saying that the Ich killed your fish directly. The high CO2 spike you had initially and the resulting large swing in pH killed some and weakened others. The Ich was just an opportunist. However, now that Ich has had a chance to flourish breifly in your tank, the water/substrate is most likely full of parasites. The more free-swimmers you have, the more likely it is that even the fairly healthy fish will succomb to Ich. Thus the recommendation to treat the tank for Ich. Fish can have Ich quite seriously with NO external signs on the body. It often attaches to the gills where it's hard or impossible to see.
In any event, it's really no skin off my nose if I'm believed or not. Many people have been very helpful to me here, including you. I just want to return the favor and try to help with some advice that I honestly believe will help make your problems go away.
You CAN make your current setup helpful to your plants and not damaging to your fish without a controller... and even turn off the CO2 at night... Provided you don't try to "draw down" the pH too much with the added CO2 when it *is* turned on.
I've got a Pinpoint pH electronic meter that doesn't have a probe. It's not a CO2 controller, just a pH meter. I'm pretty sure it still works. If you can get a probe, I'll mail you my meter to use for a few months. Then you can really see what's going on with pH/CO2 in real time. If you decide to get a controller, the probe you get will work on it too.
If you want to really push up the CO2 (and consequently really draw down the pH) then I would strongly recommend a controller.
Best of Luck...