That’s a fair point, although not much different from a traditional culture. Any culture will have a trace of nutrients at all times although the fertiliser may be the biggest difference, in my culture I use a fertiliser with less phosphates in comparison to other brands that have a higher NPK ratio. F2 for example don’t give you a ingredient list and you won’t be able to have a idea of the N to P ratio or concentration of the product not to mention that some may have silica which is not needed to grow our phytoplankton cultures, Guillards formula was developed to grow all kinds of phytoplankton including diatoms that need silica. So I’m a way if you are using f2 with the original formula you may be introducing silica to your tank as the stuff we grow don’t use it.
In addition the fertiliser I use in mine is from organic sources of nutrients in comparison to the usual fertiliser that are made of inorganic No3 and Po4, micro and macro algaes have to use more energy to assimilate those forms of nutrients therefore using a organic source of nutrients gives me a more concentrated culture as the phytoplankton don’t have to waste energy in trying to reconverte nitrate into ammonia, the benefit is also that any of those organic nutrients escape into the tank they will be a good source for coral as many will know most corals that we keep are photosynthetic and they will use those nutrients to grow before being oxidised into nitrate.
so in conclusion you really should look into your fertiliser as that’s the risk in both methods. Using this method I’ve also made a thread we’re I used phytoplankton and this fertiliser to lower inorganic nitrates and phosphates in my system wile replacing them with more desirable nutrients for coral and filter feeders.