One plant is doing great, the other is smaller and...

I think they're looking good, for only 3 weeks old their size is not alarming, its completely natural for a plant to do better than its companion. I see you mentioned the use of the ocean farm soil which is very good and should have enough nutrients for several weeks. They may just be having a small recovery time from the transplant which can slow things down temporarily. I would just watch your waterings and keep them as evenly moist as possible and then allow them the several days to dry out, also if your humidity could go up slightly towards the 60-70 range that'd be probably good also. Being patient is one of the biggest struggles i have, but it tends to work itself out just giving them time and waiting. Happy growing. ✌️

hey there Dr.A ! a few things are combining and leading to your issue. First off overwatering, how can you tell you're overwatering ? the puffy leaves and the puffy space in between the veins of the leaves are a tell tale sign you're overwatering. I see you're adding cal mag but i also see what could be calcium deficiency, why? well calcium gets absorbed at <6PH something like 5.9 so you actually want to feed at lower PHs and let your soil buffer it out. PH of rain water is 5.5 does that make sense? Other things might have been hindering the development of these plants for example the solution temperature should be closer to 21 °C so it doesnt shock the roots with cold. Last thing is, looks like a combination of calcium and phosphorous deficiency. P deficiency leads to lower leaves to darken, get spots , curl down a bit and get yellow eventually. A common symptom is red or purple stems... basically exactly what you're seeing. Not to worry very easy to remedy , just get some bloom nutrients and start giving it to the plant, even before the flower, and the plant will recover. Hope this helps Dr.A ! 🚀