Hello,
Refrigeration allows the phyto to last longer as it slows down its need for food. It is possible to keep phyto alive for more than 6 weeks in a fridge, but it does start to die off after a week or two BUT dead phyto can still be good phyto if it’s been refrigerated. Meaning, 3-6 week old phyto is still good to feed your copepods and does have nutritious benefits to your tank in general. We wouldn't recommend dosing 2-4 month-old phyto to your tank as it does loose nutrition as the weeks go on after it dies and decays and that could cause some balance issues in your tank. We feed phyto that was refrigerated greater than 10-18 days directly to our zooplankton, they do great with live or "freshly" dead phyto.
We have taken phyto that has been refrigerated (forgotten about) more than a month and been able to re-culture it, BUT lots of sediment and straining needed after a few days to remove the dead cells. It really is not recommended, and we did it as a TEST but in general most cells will die as result of lack of fertilizer after a week or two.
As already stated above, any phyto your purchase and plan to store be sure to invert the bottle or lightly stir the container daily or bidaily to keep it suspended (not a violent shake to cause foam!). The longer it can be suspended the longer it will live vs collecting at bottom/settling and suffocating itself causing quicker die off.
Also, if dosing using automated doser we recommend keeping lines as short as possible BUT in the end as long as your pushing phyto through it 2-4 times a day it will help keep lines from clogging up. Once a year you "may" need or want to pull the main line and blow air through it to blast out/clean and reconnect.
If more questions just reach out, we love to talk shop!