@losangeles67 Hi mate looks to me your plant was burnt from whatever it was you fed them
Especially if it killed other plants as well. The stress caused from this has resulted in your plant not growing at a normal rate It's likely to remain this way for the rest of it's life cycle I see you have other healthy plants so I would consider Cutting your losses with the runted plant. I hope I was able to answer your question good luck With the rest of your Grow