What Am I Doing Wrong?

Temp: Around a constant 75°F to 76°F with lights on, measured continuously at the plant tops using a digital aquarium thermometer.

Lights: 18-6 for veg, 12-12 for flower. 400w MH for veg, 400w HPS for flower with two, two bar fluorescents on the sides (once they're out of seedling stage)

Strain: Unknown, bag seed from very high grade medical stuff. Fresh seeds this time, 100% germination. High Indica genes, Christmas tree shape, short wide leaves.

Humidity: Probably a little low in the winter, all good in the summer.

No CO2.

Bulbs: Replaced about once a year.

Height: I generally allow them to finish around 3-4 feet but they grow about 100% in flower stage, which would seem to be another indication of too much nitrogen and not enough phosphorus.(?)

Wow,,,not seeing a clear reason for your non finishing, finish.....I might go on up to the 600w,,,,at least it will help penetration. Cut your flower back to 10/14 after 6 weeks as that helps late running sativa's to finish a bit quicker (signals the plant the end is near so it speeds up reproduce it's self)

might be a light leak or some light exposure that keeps it slowing down....
I'll keep thinking

Thanks!

I'm convinced that others are onto something with the P deficiency. The pure perlite holds a lot of nutes and I'm thinking it's holding onto a lot of N from the veg stage. I also was mixing the GH nutes in the ratios from the bottle so they were still getting a smaller amount of growth nute even during flowering. I had a plant turn horribly purple and red once about 3 or 4 years ago, too, which would indicate too much N and not enough P. That's why I was asking about those P supplements but I had a feeling they were too strong so I had to ask here and I'm glad I did before attempting to use them.
I won't lie, it does seem pretty difficult to regulate the nute in the pure perlite. I did some research and apparently perlite holds a LOT of nutes. Maybe I should stay organic since that's how this crop is starting out so.....wood ash?

Personally I think more N in flower is best till the last 2-3 weeks.
What good does dead and full on yellow fan leaves do halfway into flower.

Yeah, that's just it, I don't get that at all, even with the much smaller amount of growth nute during flower, which would seem to indicate that the perlite is holding onto a bunch of N.

I've been using nothing but GH nutes on the plants, too, with nothing else, and I've been wondering if that might be a problem as someone indicated above.

Update:

The growth seems to have slowed down by a lot. I'm thinking of giving them a shot of ~200-300 ppm urine after transplanting to larger pots, in trying to stay with organics. Yes, I said urine.

I'm thinking maybe growing isn't for you if you've been at this for years and are about to feed you plants urine.

For what it's worth I grow with lots of perlite and use a high N fert and never have trouble finishing a plant, my guess is you get impatient at the end with your flowering time and cure. I find 9-11 weeks from the first signs of sex (as opposed to the switch to 12/12) is what most of the strains i grow in my environment take.

Why not? The NPK of human urine is around 15-1-2 and it's sterile as long as you don't have a UTI. I will, of course, dilute it by a bunch and pH the solution.
No, I'm not being impatient, I get lots of red hairs but no cloudy trichs no matter how long I wait. I had a plant start to die once from my trying to allow it to mature. I've been told by more than one source that hydro nutes alone won't cut it so I'm thinking that's my problem.
Thanks for the helpful advice but I think I'll keep growing.

PS> I've also read that others have abandoned pure perlite for their grows citing that it was too difficult to regulate the nutes.

PPS> My buddy has had the exact same problem using pure perlite and nothing but hydro nutes.