Thanks for this! I thought that might be the case with home cultures that have crashed but I have no experience with live cultures so figured I better ask.Commercial phyto for feed from a reputable source like Seachem or reef nutrition (I.e. phytofeast) works well ime. It's also way easier. Some of them sell live versions as well as preserved and it doesn't make much difference as far as I can tell.
But I expect they harvest the phyto when it's most nutritious and then preserve it for use as feed. A home phyto culture that's crashed is not the same.