Aqua Man
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Yup always phosphoric or sulfuric acid. N as you say decomposed quickly in comparison to P or S acid in our conditions. Prob the worst to use in hydro is citric as it reacts with bicarbonate and is released as co2.Looked up a little more after my last post. I was bang on, so will offer a little more 'possibly wrong' info
You will find P acid more stable than N if your hardness is carbonates. This is because only half of the P acid is available for smoking off carbon. The other half will live in an equilibrium with an equivalent amount of carbonates. It's this balance of having both there, that means a bit more of either, won't make all the difference.
Using N-acid just smokes off the carbon. Gone. Until the acid or carbonates are exhausted. They can't set up any equilibrium.
It's really not something expected to know about when I was at school. I have huge gaps in my understanding.
But I disagree here because his problem is PH crashing not raising... so it can't be an issue of the acid breaking down as that would cause PH to crash.
Imo it must be an alkalinity, buffering, stability issue caused from lack of carbonate, bicarbonate, hydroxide or silicate as seen by the rapid drop in PH. If it was an issue of PH rising to fast then I would agree look to the source of acid used.
My guess is there is some ammonium in his nutes and as the plants uptake it the ion exchange is dropping the PH drastically due to an extremely low alkalinity.