phosphate swings?

[Dk638]

hi mr, i have a 75 gallon mixed reef, i have been suspecting a low nutrient issue due to pale colors and a couple of acros stn'ing on me. my nitrates are always zero and i was testing phosphate with red sea test kit. i was getting different readings so i ordered a hanna checker. last night i checked it and it was .09. This morning it was .00. i'm wondering if the chaeto in my sump could use that much overnight with the sump light on?
all other parameters are
cal-500
alk-10.2 (always steady)
mag-1400
ph- 7.9-8.1(tested with probe on apex)
i use kalk in my ato to keep up with cal and alk and it has stayed very stable.
i took my gfo reactor off after seeing stn after gfo change.
is this possible and should i leave my fuge light on 24 hours to see what happens?
i was dosing fuel and feeding oyster feast to try to raise some nutrients and i'm running an eshopps 120 cone skimmer.
thanks in advance.

 

[Boomer]

Many possibilities here.

1. Kalk can lower PO4 soemhar, especially through skimming
2. It may be the test was slightly different, hard to do it EXACTLY.
3. These kits and meters have error factors, calle accuracy. Most are + or - 0.04 ppm. That means if the kit or meter reads 0.05 the actual level is guaranteed to be somewhere between 0.01 - 0.09 ppm .