pH swing

Do those numbers correlate with lights off? If so that could be a great indicator. Did it just start happening or did you just recently start testing it with your spiffy meter? In other words are you sure this is NEW or did you just start catching it.

 

For the most part I would say that swing is fairly normal, it does seem to follow your lighting periods. You could try to increase your Alk a little bit which should reduce the swings a little.
When was the last time you did a water change.  I had also noticed that I had a slow down in growth so that was the first thing I did.  I usually do two large water changes spaced out a couple of days.  Then test and bring measured items (cal, alk, ph, mag, etc...) back into range.  This makes sure to put bake trace elements that you are not testing for.

 

Taavis do you do 50g at one time or do you do 25g every two weeks?

I would try to step up the frequency of your water changes. do you dose with your auto top off or manually? try dosing at night (12:00-1:00am) when the PH starts to drop, this will help to stabilize PH.

 

PH=overrated

Feed more! I started slacking on feeding and noticed a huge change. Now I am back to dumpin' in the food and all seems well.

Also, Do smaller water changes more frequently.