That should help introduce some bacteria to the system. However, it doesn't mean you have enough of NH4 and NO3 reducing bacteria to have a fully cycled environment. Most commonly NH4 consuming bacteria are present in the aquarium but not NO3 bacteria. They will only be present in anoxic zones one parts of your filter media or deep in fine substrates. These anoxic areas are usually deep inside larger sized bio media at least 1/2" in diameter. There is nitrification or the reduction of NH4 to NO3. NO3 is less toxic but still toxic at elevated levels. Then you have denitrification which is the reduction of NO3 to NO2 gass that will leave the aquarium through glass exchange on the surface of the water.
I don't know what brand of bulbs you are lighting with, spectrum, and overall light over the tank. Let me know what you are running and for how long. Then I can give you my opinion.
CO2 is jumping the gun IMO and not the step I would take especially in a new tank. You have no baseline for what your substrate+fertilization+lighting can produce. When you can keep plants alive without CO2 in your tank after it has cycled then you might want to try some fertilizer. Then just wait and see if you are happy with your growth after a few months. CO2 is for the advanced aquarist. I have had aquariums 30 years and I don't feel CO2 injection is necessary nor do I want to deal with all of the implications and common problems people have with CO2. I have a dirt bottom low tech tank man for life. I have never had to use CO2 for growth or to manage algae, the two most common reasons inject CO2. I have been very tempted to try one of those new stainless steel DIY CO2 rigs available on Amazon. I don't think the juice is worth the squeeze. If you decide to use CO2 remember that to do it right you are looking at, at least, $300 to get started. That's buying a quality regulator used, a tank used, and cheap diffusor.
The reason so many new hobbyists have problems with planted tanks is because they don't have enough knowledge to make informed decisions before they start. They see a tank that wows them and they lose sight of how much time that person spent learning before they start anything. They inevitably rush things or outright cut corners and its a snowball of issues that lead them to think a new gadget can solve the problem their lack of preparation created.
Bump: Oh yea KH at zero is because something in the substrate is leaching acids into the water column. Acids consume carbonates and bicarbonates that are measured when testing for KH. I am hoping you didn't start with zero KH. Get your KH up and your pH will be more stable until whatever is leaching acids stops leaching them into the water.
I appreciate all of the information you've provided me. Definitely several things in there I didn't know.
My light for this is a Current USA Freshwater Plus LED, and I'm not totally sure of the PAR value at this point, but its on the lower end. I kept it at the settings I had programmed for a similar tank prior to this- java fern and crypts, mostly. Didn't expect to have any issues.
This whole thing has just been a bit strange to me. I've kept tanks for about a decade now, and while I'm not on the same level as you clearly are, I felt I had an adequate understanding of how the nitrogen cycle, etc. works. I've got a 150g running CO2 through an inline reactor with pendant lighting that I've had minimal issues with. I rarely have to deal with much algae and my plants are doing great with the GLA ferts I use. I haven't had a fish death in there in over a year.
I set this 50g up months ago. I used a sponge from the tank I just mentioned to jump start the process. I tested it periodically, and it was set. I was even using it as a bare bottom tank for a while to keep feeder shrimp in. Then, I finally added the stratum with RO water, remineralized it with salty shrimp GH+, and waited another couple of weeks before I planted it. All of this according to research I did mostly on this forum to set up for Caridina shrimp.
I don't know if I'll ever be certain of what set everything back. Perhaps the buffering from the Stratum harmed my bacteria population. Maybe something about abruptly being swapped from tap to RO (the tap water here has 350 TDS and somewhere between 12-15 GH, and 10-12 KH...it sucks). In either case, I didn't know how my tank suddenly went from being cycled to not, and I didn't really grasp why my plants were having issues with it- I've also planted other tanks before the cycle was complete and never noticed a problem. So, lesson learned, and now I know a little more about the hobby.