I've run the tests to determine if the fly screen or tubs were causing the problem. I added a few litres of pH 7 water to an empty tub and left to sit out. Checked it a few days later. pH looks the same. So I added the fly screen to the tub. Left it for another few days. pH still looks the same.
Conclusion - I was tripping.

Could light exposure or aeration have caused the pH to go up?

I'm currently abandoning any attempts to grow from seed in this setup. I plan to germinate and grow the seedling stage in styro cups with a 50/50 soil/perlite mix. I will then either transplant or take cuttings for the hydro system.

Also thinking of changing to a small DWC setup using 1.3 Gal buckets. Bit risky considering this would be an untested system. I either do this or attempt to lightproof the existing setup. Going with DWC sounds like more fun and will probably be easier to maintain than the current system once light proofing is added.
Questions - Would I need to wash/flush the DWC buckets very often or at all? Sounds like a lot of work lifting all the roots out once a week. I also plan to incorporate a siphoning tube. (BTW this would be a non recirculating system).
Also, any downsides to using a bubbling bucket type system? Is the bubbling loud? Would a standard twin outlet air pump sufficiently aerate 2 x 1.3 Gal buckets? Can the bubbles damage the roots? (I'm planning on using bendable/flexible bubble tubes rather than air stones.)

L-dubbya, Gen, Z - any input would be greatly appreciated.
elcheapo Reviewed by elcheapo on . pH problems - What to avoid? What products/components commonly used in a hydro system cause the pH to spike? What are your experiences? What should you avoid or not do? My understanding is that food grade plastics are better. And I've noticed a lot of people use Rubbermaid tubs. Unfortunately, where I live that brand is not available. There are currently no plants in my new setup, but after adding tap water at around 5.5 (yellow) to the system and running it for a few days it has a ph of at least 8 (blue). My first Rating: 5