the red stems on this is good and the purple stems are good too but if they aint all the same color you should definitely check the new fish in sea and check out the soil the pH everything to make sure what is going on
Sometimes it just happens , its often calcium deficiency thats why the stems turn red/purple . You can fix this by simply adding a little bit of CalMg to your nutrient solution mix . Another symptom you might see is yellowing vains in the leaves that is also Cal Mg deficiency more magnesium tho . Everything in your grow seems to be fine and it looks like your almost ready to harvest ! I really wouldnt worry about it to much at this stage but it's always something to improve on in you next grow's ! I hope this information helps you to understand what the deficiency's look like and how to correct them . -Happy Growing!
Red / purple stems can be many things: genetics, low temperature, Magnesium deficiency or phosphorous deficiency. In my case it's usually a case of magnesium deficiency.