I’ve done Barney’s seeds once before (outside ...

No you don't need to supply extra CO2, as long as you renew the air in the tent. As long as you are exhausting the air in the tent, you will be fine. Also, 1.2 EC (600 PPM) is going to burn the seedling. Stop feeding and let the plant feed itself from the soil.

You dont need it. Some growers do it (you can find a lot of diy Solutions with Google or YouTube). I believe they only bring a small effect because they dont producer enough co2 + you lose most of them by blowing air out. If you want a real effect you need pur co2 from co2 bottles and special equipment to control the amount(co2 controller and a special environment to keep the co2 high.) Hope this can help you, but feel free to try it with a cheap diy Solution from YouTube or somewhere else to make your one experience, it will have no bad impact 😁✌️

CO2 is actually pretty important during veg, but DIY is pretty complicated to get it right. basically the idea is that yes, plants do use CO2 and benefit from it , but you need something like a 100L tank of DIY yeast and sugar recipe for a 1.2mx1.2 space , and if you have exhaust, maybe that's still not enough. At any time the yeast mixture can basically turn rotten and ruin everything in the surrounding area... it's sensitive, you might save money buying a system if it doesnt destroy your grow, know what I mean? anyway it probably will still not be enough even if you buy the bottles... Whatever you decide, i've linked some recipes in the comments. hope this helps ! 🚀