Bill,

Algae won't kill or harm your fish the way overfeeding or poor water quality will. Please resist the urge to add more nutrients in to your pond to bolster your plants. Your water quality is good- so don't let that become a problem. String algae usually grows best in clear water because it doesn't have to compete for nutrients with the one celled free floating algae.

I suggest adding water celery, frog bit, or water lettuce. You need more shade in your pond. What type of bio-filter system do you have? I am curious because other posters recommend adding more bacteria to the pond. The bacteria needs to be added only once a year or not at all. It will colonize on your bio media, rocks, your planting containers. It will not colonize on your filter mats for very long- you either clean your mats and wash the bacteria away (even with back flushing) or you leave them dirty and they clog or become overwhelmed with anaerobic bacteria.

String algae is also a passive filter. Try taking a rough stick (some people use a toilet wand) and twirl ithe string algae like cotton candy. There goes the string algae (and maybe some snails,and lots of little debris).