1st grow and clueless. Purple Kush/Pics

This is my 1st grow....blah blah blah. I've never done this before but I spent the last 10 years reading about it. I'm looking for a little assurance that I'm not doing anything wrong. Anyone see anything I should be concerned with or change? Anyone grow this stuff before? I have a few more pics that I'll post. Thank you for looking.

400w hps
20 Gal Reservoir / 70-72 degrees
technaflora nutes 350ppm
Temp:77-78 with light on
67-70 with light off
pH: 5.5ish - 6.0ish

here are a few more close up pics. what do you think.

Oh... I'm on DAY 19, from seeds. Planted 7, these 5 survived.

Should I be concerned about the 4 fingered leaf? I've never seen that before...

The smallest one got nute burned when it 1st went in the system, you can see the spots on the bottom set of leaves.

GO EASY ON THE NUTES!!!!!!!!!!!!!

Looks good but I would lower the temp of your tank from 70-72 degrees to 60-62.
Each level of your growing plant will have more leaves untill the leaf is fill size. That is normal

how do you keep the reservoir that cold?

I know that they're going to grow at different rates, but one of them is at least 3x smaller than the others, is this from getting burned with the nutes? will it slow their growth that much?

How do you keep the water that cold in the reservoir?

Is it normal for plants to drrop a little before the light goes out and perk back up in the dark? My plants seem to do this almost every day...