clay pebbles veg issues

really slow growth; 20/4 veg cycle, with 4 water cycles of 15 mins; set 1 hour before lights off, one hour before lights on, and then the other 2 watering cycles spaced out evenly between those two (being about 6 hours apart). They are in 2 liter pots with 1" rockwool starter cubes. only 2 have shown somewhat vigorous growth and i am really hating my first hydro setup. ANY HELP WOULD BE GREAT! really thinking about going back to soil but i've invested so much time and effort getting to this point.. what are your flood cycles like? yes I use a PH + ppm pen, keep my res between 68-73, and I also have ample light ( under a 400w HPS with light mover and 4x 4foot t5's) humidity stays above 50% and other than that the only issue i might be having is how the rockwool cubes sit into the rockwool cubes. some are always bone dry and some are always wet; could this be it? i was thinking of leveling off the bottom of my tub and seeing if i can fix it so each 1" rockwool starter cube sit around 1/4" into the water. I'd like to know what others do; or any links to grows that are sort of like mine.

Sorry wish i could help more but i noticed noone replied so ill help best i can, ive never grown hydro, seen a few, but this is a site ive seen thrown around a few times, literally the best i can do sorry. But some help is always better then none.

http://howtogrowmarijuana.com/hydroponic-marijuana-guide.html

Your drowning them. First of all 15 minutes, really? When I flood my tables my pumps are on 1-3 minutes depending on the table size and the age of the plants. They usually fill to about an 1 inche deep and then drain.

Second and just as important, you do not flood while the lights are off. I have my lights on 18/6 after 22 years of growing I've found it really is the best. So I flood three times 6 hours apart first flood right when the lights come on. Then when I switch to 12/12 I just knock them down to two floods a day 6 hours apart.
Your last flush should be at least 4 hours before the lights go off. Otherwise your over woatering and over feeding and you'll cause mold. Hydroponics is way better than soilIMO but you have to learn to use it properly, EVERYTHING is diferent. Your pH should be 5.5 not 6.
One more thing if you just have them in 1" starter cubes they probably only need one flood per day rockwool holds a ton of water.
And that res temp is a little warm.
Sorry to point out so many things but if you can fix all thos things it really is the best way to grow. I grow vegetables in my yard hydroponically. I greww 50 of tomatoes last year in rockwool slabs under some plastic sheeting in my back yard, this shit works.

word guys thank you so much. so i think the bluelab ph is broken i went to clean and recalibrate and it keeps reading over 8.0 in the 7.0 buffer and says error.... so i either need a new probe or a new pen. I think im going to buy the ph drop test kit for the time being and do everything you said there. I Will lower the res temp and also make sure that the rockwools are put in there good. I was hoping for more flood times because its rockwool in clay pebbles; and i figured the pebbles dry out. would it maybe be better to do an aeroponic clone so i can go straight into the clay pebbles with it? thanks for your input guys that shit helped alot i was really thinking about going back to soil.

echlectica - Could you elaborate on your Rockwool placement? I'm using Rockwool in Hydroton and right now have it flooding 3x a day, lights running 24 hrs, and the water level reaches about 1/2 up the Rockwool. I feel that the Rockwell is draining well but could be holding onto a little of moisture inside... My plants are only 3" tall thus far (grown from seed), but am I drowning them/or will this drown them in the future? I'm at 840ppm right now, and this is Day 14 since I first germinated the seeds.

Any help is appreciated.

I am using SuperGlow LED and a MegaGarden ebb+flow setup, 7.5 gallon reservoir

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