It is nano, and I use RODI.Sorry to hear it, that sucks. The cultures look about to crash honestly. I will try to offer suggestions but keep in mind I'm mostly guessing based on my experience. I'm just a fellow hobbyist like yourself, not an expert.
Where did you get your f/2, and how much did you add? If you don't use enough fertilizer the culture can run out and turn yellow. But usually in my experience not so quickly. If you used the recommended amount of f/2 then running out in 48 hours doesn't make alot of sense to me.
Contamination is another possibility but again, it's odd to happen so fast. Usually when setting up a new system with new equipment contamination isn't an issue. Contamination is my biggest problem long-term, but not at first and you have a brand new setup so I wouldn't expect that particular issue to pop up. Did you use rodi water for your salt mix? Hopefully you didn't use old tank water or anything like that?
I will say that your 24 hour growth was very surprising to me - phyto can grow fast but that's really, really fast growth for the first 24 hours in a brand new setup. Usually phyto takes a couple days to get used to a new bioreactor, at least in my experience - and then growth explodes after it's more acclimated.
Is this nanno? I recommend looking under a microscope to see if there is contamination, or if the cells look funny in any way.
I've attached a pic of some of my nanno for reference. I recommend looking for contamination - basically anything that doesn't look like the nanno. Big cells moving fast like ciliates or very small cells like bacteria. Anything that doesn't look like this, basically.
I did notice a few things.
I was recommended 1ml f/2 for 2L, however the bottle says '20 drops' per liter, which I just measured to be just a bit more than 1 ml.
So per direction I've only added about half.
I also seem to have a lot more light duration than I thought, running just 2 hours of darkness which I also suspect to increase the temperature a bit to high.
I've now added more f2/2, set the light duration for 16 hours a day, added a carbon filter on the air tube, and a fan to remove some of the hot air.
Not sure if I can save this batch, but we'll see.
If not I guess I need to try again.