KH & PH dropping between water changes

[tnnlynch]

I recently noticed in my 29 gal planted tank the PH was dropping from 7 to 6.4 and my KH was dropping from 80 to 40 in 7-10 days.   The prior month it had been fairly stable with PH of 7.2-7.0 and KH of 80 (climbed to 120 when I was on vacation & no water change).  NO2 is 0, NO3 is 25-50, GH is 300.   Tap water is 300 GH, 7.2 PH and 80 KH.  The fish don't seem to be in stress but I was confused as to why all of a sudden this was happening.

I have been keeping a spreadsheet of various aquarium activities by date since I started the tank in May.  Looking back it seems my PH and KH have been dropping since I started feeding a home made 'snello' for my snails and shrimp.  I generally feed the "snello" once a week after a water change with the exception of last week where I made a mistake and feed twice (PH hit 6.4, KH 40).  This jello mixture has quite a bit of Calcium Carbonate in it which I am assuming is lowering the KH.

I am thinking I will change the feeding schedule to be two days before the gravel vacume/water change to limit the time the remnants are in the tank.

That make sense or am I still missing the culprit? 

 

[Tihshho]

Plants will eat up the minerals that make up your kH overtime, that's what makes consistent water changes key. As for the drop in pH, are you injecting CO2 or using buffered substrate?

[tnnlynch]

No CO2.  Substrate is eco-complete under gravel.  Easy Green 3x week, Easy Carbon weekly, Easy Iron 1 every two weeks and flourish potassium 1 month, No significant change in plant mass lately.  I tend to prune ever other week when I change water.  The tank was running fairly consistently from May through July with my Valisnaria and dwarf lilly having filled out late July when I added root tabs for them.  No significant algae issue other than some minor hair algae on the Vals.

[Mmiller2001]

Seems mostly normal to me and adding carbonates to water would increase KH, not lower it. 

[tnnlynch]

Ah. I had it backwards, thanks.  

[Biotope Biologist]

Don't forget plants respire at night! Commonly overlooked, but CO2 is an acid and is intrinsically linked to carbonate buffer equilibrium as it makes carbonic acid. Everything seems normal to me. Fish can very easily handle +-0.5 swings.