Probably to make money off unsuspecting customers. Lots of big growers flush, so it appears to be profitable to market stuff to those unsuspecting growers.
Thanks for all the information yall. I was thinking of doing a slight flush with my first grow and now IM NAT. Seems to be bro science based off responses. Odd that so many big growers do it, but I've seen "successful" people display ignorance many times and can believe it.
You do full change outs? When i read under current’s instruction manual it says to do 50% change out usually. I dunno which to do cause i am new. Thanks