I think your comments about the ideal temps for our plants are interesting. I agree on the water temps, but very few can keep it that low without chillers. You are in the minority as far as your growing temp recommendations. Most seem to like hi 70’s to low 80’s F.
I suspect if you didnt keep your water temps that low you would be singing a different story about that pond/fish water! Talk about loaded with bacteria from the get go! That would explode at higher temps. 
Plants look good, so its obviously working for you