Co2 ph KH confusion

New to an aquarium and planted tank. Having a difficult time understanding measuring the co2 and relationship between KH and ph. My normal ph is 6.8 from tap. My KH is 3 degrees measured with API kit. After injecting co2 my ph drops to 6. Is this a bad thing? I do have a bag of coral in the tank to help buffer the ph. My question is am I injecting too much co2? And is this hurting my fish? No one is showing any signs of distress and everyone is eating normally. 20 gal high tank 78 degrees 0 ammonia 0 nitrite and 10 nitrate. 3 platies, 4 guppies, 6 neons and 3 ghost shrimp.

I'm basically wondering if I should stop the co2 injection or lessen it since my ph is swinging almost 0.8 ph. My understanding is a 1 degree ph is normal but I'm confused on how the KH of 3 degrees plays into the relationship.

 

What is your substrate and what else is in your tank? What lighting, which plants? Did you aerate tap water sample before measuring pH?

 

Eco complete substrate, besides fish I've got 3 Anubias nanas, 1 Java fern, Java moss, 2 Amazon swords, dhg, 2 wendtiis. Lighting is planted+ on for 8 hrs a day. Didn't aerate the tap water however measured tank water with no other treatments than prime after 24 hrs and ph settled to 6.8

 

Your tap water is quite acidic. I bet it is soft, too (not ideal for the livebearers).
I am not familiar with Eco Complete, some people reported higher pH after using it.
The only plant on your list that truly requires CO2 is dwarf hair grass. Being a carpeting plant, sitting low on the substrate, is not doing well in deep tanks.
I don't think that pH swing because of CO2 is stressing the fish however, a pH of 6 might have an impact on nitrifying bacteria. And that's not a problem either if plants are actively growing and the tank is not overly stocked with fish; plants alone can easily take care of ammonia.
I would increase the KH (and pH) to a minimum of 4 dKH using Potassium bicarbonate (rather than Baking soda); a higher KH limits the pH swings, allowing to inject more CO2 without a big drop in pH. If the water is too soft (low GH), use the crushed coral to raise both GH and KH.

 

I have crushed coral in the tank to help with GH and KH. So you're suggesting then to raise the KH by 1 degree? It's currently 3 degrees when I test it. Funny enough the only plant looking green and good currently is the dhg.