Hey mate!
Yeah, that's right, BioBizz suggests to switch from Fish Mix to Bio Grow two weeks befor Flowering as Fish mix accelerates the plant's growth and makes it stretch.
You don't really want that in flowering, that is why you should switch..if you have ran out of Bio Grow and the plants are craving for nitrogen go ahead and give them Fish Mix if you have Bio Grow give em that.
What i use to do is feed them well with N (Nitrogen) until 5th week then i start feeding more PK (Potassium and Phosforus (Bio Bloom or other boosters) and stop administering Bio Grow ..
Hope i could help..good.luck 🍀 with your grow!