@Rafik yea, I think the key thing here is finding whatever is growth limiting. Basically you're going to have a growth limiting substrate - something that the bugs are using all of, so they can't grow any more. Your EC probe will tell you that the overall mix has a certain amount of ions in it, but it won't tell you which ions. If there's a fairly high concentration of sea salt, that's likely to be a lot of the conductivity.
While you could attempt to measure each of the ions individually, that's not going to be cost effective. If this was a commercial operation, you'd probably want to make your most expensive input (per unit biomass output) the growth limiting substrate. That will both minimise initial opex (as you're not putting in an excess of the most expensive ingredient) but also put a selective pressure on your culture to cope with less of the most expensive nutrient.
If you're adding bulk trace nutrients, rather than adding them individually, the growth limiting one would be the one that I'd measure for. That said, traditionally your trace nutrients wouldn't be either growth limiting, or most expensive. It would normally be something like the carbon or energy source. In this case CO2, or light.
So, I'd try reducing your overall trace nutrient mix until you see a drop in productivity, then increase it again until you see no more increase in productivity (and you know it's in excess), then try turning down the light until you see a drop in productivity then increase it again until you see no more increase in productivity, then turning down the aeration, etc. Look at the cost of each and ensure the most expensive one is growth limiting.
If it transpires that the nutrient mix isn't your economic pinch point, then add excess, and control it based on EC as @Gerrit suggests.
You'll only have an issue if one of your trace nutrients becomes growth limiting. The concern I have is that if you're recycling your growth media, then if say the sodium chloride isn't being consumed by the bugs but the potassium nitrate is, then over time the potassium nitrate concentration may drop even though you are maintaining the EC. I'm not sure how you address this other than by repeating your growth limiting checks for each of the trace nutrients individually, or measuring everything.
At small scale, I imagine you'll find that wasting trace nutrients is more economically viable than maximising recycle and controlling them to perfection. Conversely you may want to measure everything, so you can set up a digital twin that can then scale up to fully optimised production that minimises all waste.
