Thanks for your reply.
After a lot of research and testing - it turn out that the PH Down I was using was too "weak" for the job.
I got distracted measuring the initial PH of my RO water as it was not making sense. I was told by people I trust that PH of RO water was unstable -- but they failed to explain further. Here is my RO testing:
Tap water PH (from the water autority in Key West) is between 9.0 and 9.5.
I measure 9.0 using a Bluelab
pH meter and PH indicator solution.
Freshly made PH comes out at the same same PH -- 9.0.
If I let it set in the open for 2 days -- the PH drops to 7.1
I made two 4 gal pails of
nutrient solution to add to my tank: #1 pail was 2 day old RO water with a PH of 7.1 and #2 pail was newly made RO with a PH of 9.0.
After mixes the same nutrient mix in both -- the resulting PH of BOTH pails were exactly the same -- at 6.7.
I have multiple books by Howard Resh. Opened then and researched PH management - specifically PH Down. I was using citric acid I purchased from my fertilizer manufacture but from previous post -- you can see it wasd ineffective. After talking with the manufacture, they stated that several of their customers stated their PH Down was too "weak". The manufacture suggest a stronger acid -- battery acid.
Resh also stated battery acid but also recommended Muriatic acid. Home Depot sells a gallon of Muriatic Acid – Hydrocloric acid – 31.45% for $6.
The solution in my tank was at an EC of 2.0 with a PH of 8.1 (about 14 gallons of solution).
I added the two pail of nutrient solution (8 gallons) to my existing tank and the resulting EC was 2.4 and PH of 7.0.
Added 11 grams of muriatic acid which brought the PH down to 5.9. I let this circulate for 12 hours. Resulting PH ended at 6.7.
Added more muriatic acid and have stabilized my PH at 5.7.
I don't expect to see a large PH swing now -- but the next 36 hours will tell all.
Thanks for your comments.