I suspect that nutrients are quite different than CO2. Only plants with CAM metabolism can store C at night (think succulents, but there are others too). There are aquatic cam plants (e.g.
this work). So, those plants are taking up CO2 at night, but supplementary dosing is still probably unnecessary because of the higher CO2 available from the continuation of respiration but cessation of photosynthesis. However, most plants can and do store nutrients, some even being luxury consumers that can store quite a lot. That process is independent of photosynthesis, however it may be dependent on the resistance of diffusion and thus could be impacted by any stomata and their closure. That said, I presume vascular plants are likely to be able to store higher levels of nutrients, which me be able to be be done at all hours. If this hypothesis is correct, then it would seem to me best to dose in the evening or even after the lights have gone off. It seems clear that algae are faster at taking up N and P from the water column, which is why I was wondering if they experience a trade-off with storage capacity, and thus timing of input might favor vascular species.