Leaves yellowing

I am having issues with my plant. The leaves are yellowing between the veins. Its about 23 days old. PH is 6.8-7.0. 400HPS about 3.5 feet from plant. Can't keep the temp below 85. Any ideas what this is?

Classic early symptoms of a magnesium deficiency. Good eye, you caught it at a good time.
Make a solution of 1/2 teaspoon Epsom salts (MgSO4) in a gallon of water. Use this as a foliar feed.
To prevent the problem from recurring, get your soil pH down to the 6.5-6.8 range.
Also, avoid overwatering. I can't really see the soil, but it looks unusually dark- check to make sure it is not soggy.
Your next watering, adjust the pH to 6.5 (use cider vinegar if you have no other choice), which will start to lower the soil pH a bit.
Micros get locked out above 6.8 and below 6.5.

I am foliar feeding the solution of epson salts and water as suggested. The plant is in a 1 gallon container. I just bought a 5 gallon bucket to transplant it into. Should I wait to replant or can I just replant now, the plant is 13" now. If I did do a replant, the 5 bucket container would move the plant closer to the lights because the lights are as high as they go. Would this be a problem? Also, I only have about 3.5 feet of growing room, at what height should I force it to flower at 12/12? This is my first grow so any help on what type it is or suggestions are appreciated. I am growing in a closet with 400W HPS about 3ft from the plant. The temp is around 85 sometimes gets to 87. I can't do much about the temp, I have fan circulation and am not willing to put a hole in my ceiling to vent and the air in the house is set to 80 degrees.

Can someone click on my pictures and advise if I do indeed have a Mag deficiency? I have been feeding it epsons salt and just want to make sure that this will work. Its a nice plant and I don't want to lose it. The problem is most evident in the picture far to the right.

You do, and it's a classic one.
Nice blankets BTW.
You can wait a little while to transplant too, or go up to an intermediate sized container, like a 2 gallon.
Max tmeps of 87 degrees are not dangerous.
How is your soil pH and moisture? These are the SOURCE of the problem.

just for assurance, Stinky is correct, she knows her stuff ... it will take a few days for the foliar-fed magnesium to work its stuff ... :smokin:

Just for clarification... I have the analog dial type PH tester. It basically sucks. Its anyones guess what the PH is, it shows between 7-8 I need to pick up a digital one today. Would it be okay to just replant it now with correctly adjusted ph?

I have always trusted liquid pH indicator. I work in a lab and those cheap pH meters bother me irrationally, lol!
If your pH is above 7, the root (HAR HAR HAR) cause of the Mg issue lies below the soil surface... you're quite simply locked out.
Composted humus has a pH that is buffered at 6.8... hint hint! IF you don't have a reliable meter, go for reliable MEDIUM instead!
Yes, it's safe to transplant now.

composted humus has a buffered ph of 6.8. Excuse me for being so slow on the uptake here but I am assuming I should go and buy some composted humus?

I'm an antique ... I still use the test strips ... curious what you feel about those, Stinky ... the ones I bought online were top-quality strips, specific for municipal water/waste treatment pH Test Strips ... I bought the ones that range from 5.0 - 10.0 ... wide variety of test ranges available, though ... :smokin: