Hey Pal,
Think it's an controversial topic. Some say so, the others so.. I personally don't flush, I have an additive to get rid of unwanted stuff.
Then just plain water, pH'd..
At the last feeding I add some Humic Acid to improve the taste..
(For your info I'm growing on soil)
I'm not sure if you will find solid sources about that.
I've seen that some of the breeders (RQS for example) have additional Information about such topics..
Or Zamnesia, they also have a big Blog Section.
The 48h of dark will not increase the THC, or other fairy tales. But it helps maturating the Trichs which are clear when you want to go for the chop..
So after an amount of time (Input mine for 36h in the dark) the clear trichs will have turned milky..
Hope this helps a bit, good luck and happy trails for the rest of the way.
Cheerz✌️
OimRausch