Help DWC, seedling stunted !

Hi everyone,
It's not my first hydro, but it's the first using Ro water.
I moved to an other "city" and tap here comes at 500ppm and ph at 9+ and even after let sit 48hrs, still smell chlorine or something like this..we don't drink it...
anyway, I invested in Ro machine; now water is at 15ppm (more or less)
started seed supercritical from GHS
Germinate the seed, and placed in a 5gal bucket
ph test kit drops yellow/orange, around 5.5/5.8 and steady (need to change the probe of the meter...)
it has been in the bucket for 2,5 weeks now, and very slow growth, ugly seedling ....
my temps are 72/74
and rez temp 62/64 (I let the pump outside the tent, maybe too cold??)
My light is a 300w GSL
I have reading no nutes until 2 weeks so it was at 130ppm ,I have drop it to 70ppm yesterday but it seems very low..there is roots popping out the 8" net pot lid nice and white..
I have others plants in soil that are water with bottled water and are doing awesome ....
First time I am facing such issues, and I am very disappointed....
guys, what do you think???

what kind of nutrients are you giving it? looks healthy but id be giving it 300-400 ppm of like GH floral series

I am giving for now Canna aqua vega A/B very low dose (70) after thinking that starting at 150 was too much...
Do you think she is starving?

the popping roots showed in the photo has grow like 2 inches last night, very nice, but no vegetative growth..
I have put some Ams from seed a week after in a small tub DWC and they are developping healthy roots too but very slow vegetative growth too...
to the other hand, I have also in soil 2 seedling and look super healthy with good growth and just watered with bottle water.
should I bump the nutes to 300?
About Gh series, I am waiting in the mail micro and bloom to try lucas. but I won't have it until next week .

i do lucas alot its pretty finicky if you dont have the pH right um yeah think you should raise it to 300ppm the roots are searching hard for food

ok Hexthat,
I went to bump the nutes .. 70 to 330 ppm (0.65ec) add some drops of ph up. done ph
I have take a look at canna charts and recommend 25ml of A and B at this stage for 3 gal of water...
Maybe she is really starving...will see ...
I have let the DWC tub with the 4 seedlings of Ams at low ppm to see if there is any improvement in the bucket before...Hope I don't burn it ...