Wtf is going with my res?

What's up guys and gals, havent posted in a long time, just been chilling. So today I noticed something weird, I'm running 4x4 tables right now with 40 gal res. I noticed what looked like ph issues but my res was at 5.8. I checked the water in the table and it was 4.6. As soon as it dumps back in the res it's back at 5.8. I went and got a new meter today cause I thought my old one was broke but nope, new meter reads the same. I put the water from the trouble table in another res and flooded a different table and it stayed at 5.8 for the whole process. I scrubbed the trouble table many times tonight and put the water back in the trouble table and the ph still plummets when it fills the table. I've been using tables for a long time and never seen this, anyone have any clues?

Are all your trays the same brand? By chance is the problem table an inferior brand?

I've never seen or heard of anything like this. All I could think would be an issue with the plastic in the tray, or possible an issue in your medium. Possibly hand water a plant and test it's run off.

Otherwise Im stumped.

Try to eliminate variables in your testing.... Maybe use the same vessel to contain the liquid your testing each time... Just a thought.

What is in the rez? Nutes additives?