EC going up daily during the day - Hydro recirculating system

Evaporation is a possibility. if tank seals well, this is less likely as it'll condensate and rip back down.. minimizing any amount that can drift off at one time. more fans on during light hours or do they run 24/7? That plus extra heat will likely be the difference in evaporation between light and dark cycles. where are the meters? could this impact their readings? the plant drinks... what is in solution isn't selected out or in... so the plant drinking more or less won't affect the EC. How quickly the plant drinks will impact the time allowed for evaporation to have an effect, though. Each rez change and even top-off to some extent would reset or partially reset that clock (top-off). Or, if oyu used your meters to level it off again with a little added h2o, if needed... however it gets done, lol. Rate of evaporation will impact EC because as some water goes off as a gas, the solutes (nutes) remain in the smaller volume of solution that remains in the substrate -> increased EC. Other possiblities are other life forms doing all sorts of potential things lol... but in this case i doubt it is a microbial event.