corals uptake ammonium, though at much lower rate than their zooxanthellaes uptake of ammonium. but you are incorrect that corals cannot use nitrate. they can. just not directly. corals can use both nitrate and phosphate and can survive and grow with ONLY nitrate and phosphate dosing without any other food. they cannot use the inorganic N & P directly but their zooxanthellae can and do. the coral then ingest part of these zooxanthellae and can then utilize the organic N & P for their own growth. I just read a paper on this, study in 2023 that experimentally showed this and showed that the N & P from the nitrate and phosphate got incorporated into coral tissue (not algae cells in coral tissue but actual coral tissue) via stable isotope tracing. this is new knowledge. corals do this in nature in areas with seabird colonies for example. corals ability to literally function as an animal and as a plant i.e. utilize both inorganic N & P and organic N & P depending on what the environment is providing is part of why they can survive in nutrient deserts and outcompete algae (when not stressed by anthropogenic harms and global warming). also side note acropora have been shown to feed on phyto. their zooxanthellae then consume the inorganic N & P that is left over from digestion of the phyto. corals and their symbionts cycle N & P very tightly and efficiently. the bacteria, archae, fungi etc that also live in them also contribute to this cycling. it is complicated and still being unraveled. get on google scholar and search topics you want to understand better and read the literature snd studies and that will really help give a much broader and deeper understanding of what’s going on in your tank.
