I’ve moved 4 emersed E. cordifolius plantlets to the planted tank and find out they have grown out these red leaves. Is this a defficiency leaves or they are turning into submerged form. Is it possible that they turn red cause of CO2?
The osmocote should not be used in an aquarium substrate for submersed plants. There is way too much nitrate (which these plants cannot utilize when growing submersed), phosphorus and probably iron. I am not saying this is the cause of the red leaves, just pointing out the danger. If grown permanently emersed, that would be a different issue and the osmocote would presumably be OK as terrestrial plants use nitrate not ammonium for their nitrogen.
Many species of Echinodorus sprout reddish leaves which turn green, or remain reddish, or develop reddish blotches--all depending upon the species, and the growing conditions. I would leave these and see what develops. Every new leaf my E. major grows looks like it is dead, but it turns green as it grows.