What Am I Doing Wrong??

Ok so user name says it all! Haven't grown or smoked weed in over 25 years and thought it was time to revive my bad habits. I'm trying DWC with White Widow autos from Nirvana. 3 seeds were put in rapid rooters that were in hydroton with an emitter just below the plug on 1/19 and all 3 emerged on 1/22. Water from tap is 44 ppm and about 7.5 ph. Using GH nutes and ph down + hydroguard water is adjusted to 5.8 (eases up to about 6.5 before readjusting down) and 340 ppm. Tent temps range from mid 60s at night to upper 70s when the lights are on. Water temps are ranging from upper 60s to low 70s. Light is 6 50w Vero 29s at about 20 inches in a 3X3 tent. This is what they looked like at 1 week on 1/29

Thought I was doing ok at this point. Maintained routine and in a few days noticed yellowing on bottom set of leaves only. This pic is on 2 week from emerge. 2/5

Getting concerned about the yellowing so I called company I bought components from to ask about light heights and quantity. They assured me that 20" was plenty of height at full intensity(actually could go lower) but recommended that I get an oscillating fan and direct it between canopy and lights. I already had a small clip on fan which was just stirring the air up but not directed near the plants. Put the fan in and changed out the water(1st time) and went from 3.5 gal buckets to 5 gals on Thursday. This is today...

Not nearly as concerned with the yellow bottom leaves as I am with the overall size of the plants. Shouldn't they be larger? Never heard of autoflowers until about 3mos ago(told you I have been gone a while) but seems to me if they will be done in 12 wks they should be growing faster than this.
So.....what am I doing wrong or what am I not doing that I should be?

Your PH is too high. You should be shooting for between 6.3 and 6.5

Use this chart for refference,

Water in buckets is kept between 5.8 and 6.5

Thought I would provide an update. The other two white widow autos have already been harvested-too early. Got a little impatient with my first attempt in 20 or so years- also needed some “medicine.” I think I’m going to be able to let the last one get a little closer to prime before I cut it.