Test the KH. This will help know where you are at.
But in general, EI is applied only to CO2 tanks, modifications for CO2 are divided about 10-15 x.
So you'd only dose maybe once every 1-2 weeks and then not much.
the ferts should not do any harm to ottos, I have a bunch, they have lived for 3-5 years. Which might be a long time or a short time depending. When you first get them, they die like flies, but one or two seem to never die.
Likely they get enough diatoms, while the others do not as much.
Still, the pH should not move around that much and the pH should rise during the day, and fall at night and be the lowest at pre dawn. Highest at the end of the light cycle, this can be 2 full units if the KH is low.
But fish are fine in natural systems under these conditions, so do not place too much faith in the pH are some meaningful parameter when plants and CO2 are involved.
Folks add CO2 gas to supply the plants, but even in non CO2 tanks, the plants will use up all the CO2, and that drives the pH up, but this is not the case in CO2 enriched tanks, since folks add more CO2 than the plants require, the pH drops some, but does not rise while it is being added.