I just did away with my 400w MH a month ago. I had it in my oak canopy, But I never knew if my heater in my sump worked or not, I had Six 80mm Fans running all the time in the winter. Summer is what about destroyed my tank from over heat. I give my MH system to a LFS for $25.
It will be well worth your time to lean about the 3w led.
I went over kill
, I bought 36 Cree 3w led " 24 royal blue, 12 8500k white" with the 3 meanwell 48p drivers, Then I built a Arduino controller to Ram them up, and Down. All for just under $350
Granted my arduino now even controls my Hydor koralia pumps
But ya, I was
a DIE HARD MH guy, But Back then Led's were Way out there on price, Now you can get into Leds that I think look better the what my MH did and lower cost. Just My MH bulb that I changed about every 10 months was $80 + $15 a month on electric.
But Learn what your looking for led's, there are alot of fixtures out there that Claim to be a high output but only have 1w 6500k leds installed for $299. In witch yes they work fine, not as blue as I like, but one called UFO is about the same price and 1w leds Has been on a buddy's tank and coral is growing like Mad in that 125g. Just seeing that is what made me pull the trigger and go led.
I really hope you knowing my experience can help yours be better. And ya as far as shimmer, hehehe, I have more now then before, and Lovin it !
Any questions feel free to ask
Oh ps, if you are set on MH for now, yes a small 75w - 150 or so MH with T5 HO can be done in a 4ft long canopy with fans about 12 inch off water, I have seen it done, but they also had there house set for cooling at about 78 in the Hot summer days. So Summer time and Room Temp makes a Big difference.