I have a gardenpond (roughly 16000 litres) which is crystal clear. The pond contains no fish, only amphibians and insect life and a lot of plants. some plants, like the pondweed are real hardhitters on oxygen production and using nutritions to grow. I learned that a healthy amount of plants can prevent the growth of algae, and so far it is paying off; there are almost no algae in the water.
however, there are some examples of plants growing, of which the leafs start to decompose (in spring! They should wait for fall...) and I am wondering if this is due to the lack of nutrients. I never really thought about the "rule against algae" the other way around, but it sounds logical: there is not enough food!
I did not test the water yet, and in case of another cause, maybe it is because of acids or other stuff. The overall appearance of the pond is excellent though.