NH4 is certainly a possible factor.
I do suggest large water changes as a matter of routine as it is.
Good plant growth should negate any NH4.
Plants actively remove it.

Some good rippling and more CO2 addition will help.

You loose some CO2 with more movement, but the trade off is worth it.
Then you can have good stable higher O2 and good stable higher CO2.

What you do not want is variation with O2/CO2 ratios.

Regards,
Tom Barr