nahhh 6-7 gallon is fine for finishing pots .. just repot em a week before you trow them in under the 600W HPS or do as I .. and Veg em a week under the HPS .. go down in light time over a few days from 20 to 16 and then down to 12
18 days after 12/12 begon ..
thats how much they stretch them fist two weeks or so of flowering .. from 1 to 2 feet roughly ..
so make sure you got boath the light for it and room in the pots or you might get some problems ..
Edit: a pic from day one in 12/12
hope this give you a idea on now much they do stretch doing flowering ..
tho I doubt they would have done that much under my 27900 T5 lumens as under my 92000 HPS lumens .. and I guess that is the diffrent .. insted of haversting etc. 6 oz in the end you might just get 3oz with the T5
if that is ok for you its fine .. go with the T5 only .. tho get the best once you can get your hand on if you plan on use em from start to end ..
I would look for a 8 or maybe even a 10 or 12 tube fixture .. can get somre realy nice once for 200-300$
maybe even the VHO type (Very high output) tho cost a bit more ..
and get the right bulbs as some also said .. dunno much about them for fish tanks .. some even is 12K Kelvin !
but atlest get some nice high out put 6500K and 2700K bulbs .. enough so you can runn etc. 6 6500k and two 2700k in Veg and visaversa in flowering ..
if you runn em all the time .. I would changes tubes evry half year or so .. atlest evry year .. even tho T5 last a lot longer and dont loose as much light over time as etc. T8 or T12
another good thing about spending the littel more cash on a nice grow fixture is that it also comes with some good refletors .. that also make a hugh diffrent from lets say a 1 tube stripe with out any ..
and since they dont make much heat compaird to HDI lamps you dont need to spend as much on ventilation .. just for fresh air/co2 and for odour controle .. less money .. less power use .. less noise .. that is some of the kool things about T5 ..
and ther for I would also suggest to runn en 24/7 will help or atlest as I do 20/4 if you want some dark time doing veg.