I have osmocote water garden, I should use it? https://www.lovethegarden.com/au-en...ext=A set and forget, no,for up to six months.Most likely Echinodorus 'aquatica', but there are several others with that type and size of leaves.
Most Echinodorus will thrive with very low levels of CO2, so I doubt this is the cause of the very pale leaves. I recommend adding a nutrition capsule right under the plant, when planting in your tank (do not wait much longer, doing that). That will feed this particular plant a little extra. Replace capsule pr. instructions on product.