The plants in the 5 tubs may be consuming more nutrients than the others.

The heat may have caused a reaction resulting in chemicals falling out of the solution.

Are your methods for testing PPM the same in all tubs?

Good luck. Wish I could be of more help.

DrKnow...

Thanks for responding. I'm not sure the strawberry plants were big enough to be consuming very much. But yesterday the tubs were showing less difference in ppm, although still large, running from 750ppm to 1150ppm. From that has to be subtracted the 230ppm of the tap water, which makes the difference larger, percentage-wise.

Today I drained the nutrient solution, cleaned the tubs, and switched to the two part nutrient from American Hydroponics. There's much less difference from tub to tub.

I don't know what effect heat has on solubility--at least not moderate heat. The highest recorded solution temperature was 84 degrees F. Some of that heat comes from the air pump. Strawberries don't seem to be bothered. At least, not so far.

I'll be communicating with American Hydroponics because there was some hard precipitate stuck to the bottom of one of the mixing buckets. I think that's normal, but I want to be sure.

just a little FYI, in Chemistry 99% of the time heat makes things dissolve better. your problem could be not enough movement in certain areas... if you are running 1 air pump to all individually then it could be as simple as not having enough pressure coming out of that particular stone.

Big Joe...

Everything seems to have settled down after I changed solution, with much less variation in ppm from tub to tub. The strawberries are growing well, but that doesn't seem to be making a dent in the nutrient solution concentration.The average ppm remains at about 1110, even after I added some water. I did adjust the pH downward, so perhaps that's responsible.

No response from American Hydroponics

well keep me posted on how well things work out with it, I'm very interested in actual experience from them. I'm not so much a Chemistry person so simpler is better in that sense to me.