I know this is an old post, but I am hopping to shed some insight. I was having the same issue as you for the longest time, I was starting to spend so much more on phyto then I was trying to save lol.
The issue was, you need to start with a 1:1 ratio, one same part phyto culture one same part fresh salt water. I was trying to do like 1:14 to save phyto and it would get a little darker and then die day 3 ish. Once I started 1:1 I am growing dark / dense cultures. I do however, still have to harvest day 6.