How long will this "media" deliver nutrients?

The three main nutrients in most soils are nitrogen, phosphorus and potassium. In a pond, these are always being replenished with the water circulation. Roots and root hairs help capture and pump hydrogen ions into the soil. As long as you have plants and a plant media and a pot you can contain and help exchange soil nutrients. It's all part of the Ecosystem, but make sure the soil and pot have water exchange and that you don't cut off the plants healthy water exchange. Fabric pots work wonderfully! The Pond Company ‎Processes

how many bags will you need for a 20 gallon L?

Not sure what a 20 Gal. L is, but I can tell you what I did with it and maybe it will help?! I used 2 bags of this to plant 6 plants in med. size bags in our Koi pond. It really works well and doesn't cloud up the water as bad as most I've tried!! And the plants thrived, so I assume it has what they need in it! :-) Hope it helps some! Good luck!!

It is aquatic media so we don't see why it wouldn't. It is plant, fish, animal and people safe.

How many large bags for a lotus planted in a 20 inch container?

Hard to answer without the depth of the container. If we figure the radius to be 10" and the depth to be 6", then you would be looking at approximately 4 bags.