It's not uncommon for pH in aquaponic systems to rapidly go from around 6.8 to 6.0... especially "newish" systems... as nitrification really kicks in with an increased feed load...
And most established mature and highly productive systems are commonly around 6.0-6.2....
See Joels, F&Fs, mine.... and many others...
And rather than try and hold your tank at 7.5-8.0 Boris.... you'd be better off holding it around 6.5-6.8 IMO... otherwise you'll suffer trace element lockout... especially iron....
Just test it periodically... and when it's around 6.0-6.2..... then buffer it back a bit...
Other than that.... leave it alone.... just add fish food...
