I don't think that the sugars from the fishmix are strictly necessary, it just allows the chlorella to continue growing during the dark period and they are included with a view to promoting microbial growth around the root system of terrestrial plants more than for their value as a fertiliser. It just happens that Chlorella can be grown mixotrophically and exploit the sugars directly.
I have literally zero experience with dry salts so I can't offer any useful advice there. I wish I could test my chlorella culture to see what the levels are like over time, what sort of tests are you using? are they expensive?
If there's something off about the amount of light and the size and shape of your containers is it also possible that some of the chlorella is dying in the actual chlorella culture before it even reaches the moina somehow and producing ammonia?
I wonder if
@MirandaB has experienced anything similar with the bottles she's still growing on the windowsill?
One of my backup cultures in a plastic pint glass got fed with a huge amount of chlorella and has been totally neglected since then with no water changes and has good population growth and no signs of a crash so it seems it must be possible under some conditions to purge any harmful ammonia or nitrite.
I wonder about a drop or two of seachem prime in the chlorella before feeding as a safety measure maybe?