Hello, Helene,
I have heard about this phenomenon before in other fishes. In Dario dario the sex of the young is obviously influenced by temperature and water hardness. I had no time to search thoroughly now, but I found something here:
warmer: more males, cooler: more females
softer: more females, harder: more males
Concerning the humic acids and the organic load of the water I recently had similar questions at the IGL-Forum – which splitted a thread and created confusion (which was not my fault!!). The result was a very interesting thread with extensive thoughts about black waters. I’m afraid it’s all in German (but that’s no big problem for you, I suppose)