Week 5 flower update, EC creep issue in DWC

Running a Zkittlez pheno right now in a 4x4, week 5 of flower. Four 5-gallon buckets, recirculating DWC with an Inkbird controller keeping res temp at 68F. Under a 600w HPS dialed to full power plus a 200w Samsung QB for side fill. Plants look solid overall, good bud structure, trichome development right on schedule. The issue i've been chasing is EC creep between reservoir changes. I'm targeting 2.0 EC (1000 ppm, 500 scale) going into mid-flower, but by day 3 after a res change it's climbing to 2.4 or higher without me adding anything. Runoff pH is stable at 5.9, so i don't think it's a pH-lockout situation causing phantom readings. Plants are drinking water faster than nutes, which is usually the cause, but my ambient RH has been low lately, sitting around 38-42%, so transpiration is elevated. I've been correcting by topping off with straight RO water and a tiny dose of calmag, which brings it back to 2.0 EC within a day. VPD is running about 1.4-1.6 kPa right now, which is in range for mid-flower but on the higher end. Thinking the low RH is driving excess transpiration and concentrating the res faster than normal. Curious if anyone else has dealt with EC creep specifically tied to low humidity periods. Wondering if i should tighten my top-off schedule to every day instead of every 2, or just ride it out. Open to hearing what's worked for you.

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