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I have grown many lillies and i find the best thing to do when introducing new ones is to have them so their leaves are a couple of inches below the surface then they will make their way to the surface and as they break the surface you lower them again until they are at the depth you want.
I have had 2 flowers from mine this year and will repot it in the spring with proper fertilizer and aquatic compost. Don't forget, lillies don't thrive in moving water. I bunged mine in on bricks to start with, but the leaves were a good couple of inches below the water, then once they were at the surface, I dropped it down the rest of the way. The water is now almost covered in leaves and the water is clear as a bell! I got mine cheap cheap at B&Q during their end of season sale - think I only paid a couple of quid for it! ;D
Okaaaaaaaaaaaaaaaaaaaaay, I know that, but the lilly leaves prevent the light entering the pond, as you say, stopping algal growth, thus a clear pond?