Iíd like to expand upon the good comments, above.
PH is not the evil we once thought it was. What we used to think of as pH shock was, in reality, TDS shock. This is the combined minerals in our water and is easily measured with inexpensive TDS pen-type meters. TDS is mainly composed of the two minerals that GH also measures. PH just happens to also somewhat reflect changes in TDS, but itís better to focus directly upon TDS for the impact it has on fish. Regarding CO2-driven pH changes, as @wkndracer
mentioned, they do not affect our fish in the way that TDS changes do.
CO2 fine-tuning and measuring tools can be fun to play with, but (as @wkndracer
mentioned) you donít need to be so precise and will never achieve precision, anyway, because the crude tools we use (dcís, charts, formulas, etc.) only get us into the ballpark. Here is another one that I use as a guide: Rotala Butterfly | Planted Aquarium CO2 Calculator
Most of us do try to keep ourselves comfortably within the 30-40ppm CO2 area. Generally, a one-point pH drop from fully degassed water gets you to the 30ppm area. However, the most useful recommendation is to raise your CO2 Ė gradually Ė until you see your fish struggling, then back off, let them adjust for a day and try again. Once you reach the point where they can no longer adapt, leave it there. You will find that this is the best way to maximize CO2 and usually gets you near the 40 ppm level, which is a greater pH drop than one point.
Virtually all community fish will be fine at Ďlowí pH levels and, I believe, shrimp prefer pH in the 6-7 area. Many of us run with pH in the 5ís. KH will be a big determinant of your pH level, as you can see in the link I supplied, but it doesnít affect your CO2ppm level. It just moves your pH level which, again, is not harmful to your fish. Some of us also believe that maintaining pH in the 6ís increases the ability for the plants to uptake ferts. Although I havenít seen studies that definitely support this in aquatic environments, it is well-established to exist in terrestrial plants (especially in my lawn). So, I happen to be a believer in this.
Now, the other things you mentioned: I donít like the fact that you stopped fertilizing. Healthy plants are critical to minimizing algae. Healthy plants only come from the right balance of light (drives everything), CO2 (most important nutrient) and ferts (must be non-limited). If you are having particular problems with plant health, you might want to start a new thread detailing those problems along with your parameters and dosing.