Distance of MH from Acrylic??

[Bamm Bamm]

Ok so I'm still working on my 55gallon Tru Vu aquarium and I'm going to be building the hood in the next week or so as soon as I get all the scratches out of the inside of the tank=) I made the hood 14" on my 90 gallon that is currently housing the lights that will be over the 55gallon.. My question is how high off the acrylic do I need to mount the MH's?? I'm going to be running 2x250MH's and 2x96w PC's.. Any help would be great.. I'm still not quite sure what I'm building the hood out of though..Probably Pine?? Gotta go to HD and buy some wood along with a bunch of plumbing supplies for the new dual 1" overflows and dual 3/4" returns..

[baconbits]

Sounds like a carbon copy of what I want to do. Take some documentary pictures and tell me about all the mistakes you make so I don't have to BTW does your acrylic cover completly cover the tank? MH is very sensitive to water splash I guess. I think I will slide a piece of acrylic into the hood just under the lights to completely protect them.

[Ninong]

I once asked an acrylic tank manufacturer (ATM in Las Vegas) how high above the tank metal halide lamps should be positioned and their answer was a full 12 inches. Not everyone will agree with that, but that was their answer.

I do know of several instances where 400w Iwasaki 6500K metal halides positioned only four or five inches above an acrylic top caused the tank to fail catastrophically. And you can't rely on a fan blowing across the acrylic to keep it cool because if the fan fails, the tank fails -- that was the exact same situation that caused Jim Rice's 450-gal acrylic tank to fail. He positioned 400w Iwasakis too close to the acrylic top, used fans to blow between the lamps and the acrylic, the fans were not turned on one day and the tank was history.

I think the most important thing is to make sure each lamp is over an open space. In that case you could probably get away with having 250w metal halides as close as 6" above the surface, provided the lamps were not directly over the acrylic brace portion of the top. If it were my tank, I would not position 250w metal halides any closer than 8" directly above an acrylic brace and probably a minimum of 10" for 400w halides directly above an acrylic brace. Again, if the lamps are over the opening you could probably reduce that by 2" or so.

What happens when a metal halide lamp is too close to the acrylic top cross bracing is that it heats up and expands at an uneven rate compared to the rest of the tank, thus applying stress on the tank walls. The result is usually a crack at the seam between the top and the side walls (if you're lucky) or even a rupture of the corner seam. Or both.

[baconbits]

Wow Ninong thanks for that input. I know this is not my thread but it has already diverted a potential disaster! My question is then, since my tank is out in the room and not in the wall, that means my canopy will sit at a minimum 12" off the top of my tank?!! That will look kinda silly. (not as sill as a melted tank however)

[Ninong]

Wow Ninong thanks for that input. I know this is not my thread but it has already diverted a potential disaster! My question is then, since my tank is out in the room and not in the wall, that means my canopy will sit at a minimum 12" off the top of my tank?!! That will look kinda silly. (not as sill as a melted tank however)

No, it won't look silly at all. It will look perfectly normal for a reef tank running metal halide lighting. The typical tank manufacturers' canopies are the ones that look silly. None of them can accomodate anything more than VHO without modifications.

How tall your canopy is depends on whether your tank is glass or acrylic and whether you are running 400w lamps or only 175w or 250w. Don't forget to allow for at least 1/2" space between the top of the reflectors and the inside of the canopy. And recommended distances above the water for mogul base metal halide lamps refers to the distance from the lamp itself to the surface. Most double-ended HQI metal halide lamps come in ready-made luminaires with UV shields and the recommended distance is usually measured from the bottom of the fixture to the surface. My German-made light fixture is 11" above the surface, measured from the UV shield. That would make the lamps themselves about 12" above the surface.

[Bamm Bamm]

Ninong thanks So much for the info!!!! I just ordered 2 4" icecaps fans to get rid of my cheapie $25 Marine Depot noise makers.. and I will run them inline with my MH bulbs and I think I will build a 13"14" canopy like on my 90 gallon and have the lights about 10-12" above the acrylic to be sure.. Thanks soooo much for the input.

Bacaonbits.. I have a 14" tall canopy on my 90gallon and it looks great.. the tank does stand about 6'2" though=) I have to use a step ladder to access inside of it..=)