Hydroponic nutrients have a buffer built in to them that helps stabilize the Ph. You need a minimum of 10 gallons of proper mixed nutrients with the proper water and you still will adjust your Ph twice a day minimum.

If you are using tap water do you let it sit out for 24 hours so the chorine evaporates? Is your water hard? If your water is hard are you using micro for hard water? Or are you using hard water nutrients? The buffers are different with hard water VS. RO water.

How are you using the fly screen again?

The tubs are not the problem; as long as you washed them out. A 5% bleach solution is fine and safe to use to disinfect the inside of the tub and all your equipment.

The Ph will change all the time as will the TDS from what the plant uses up and discards back into the res.

The swing your friend is having is something else. He may have a problem with a plant or too. Smell the roots and see if you have problems with them. If there is an odor then you have a problem with that plant.
Zandor Reviewed by Zandor 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