Thank you for sharing your question about fertilizer for aquatic plants.
I recommend using Scotts Osmocote 250g Water Gardens & Aquatic Plants Controlled Release Fertiliser. The nutrients from the tabs are slowly released as plants need them. It prevents nutrient build-up in the water and minimizes the risk of algal blooms. Please make sure to read the instructions on use on aquatic plants. Please keep us updated, any progress or photos you can share about your project would be much appreciated.
If you need further assistance, please let us know.