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Re: MH lighting for 125 and 180 gallon tanks

I have read info written by several respected members of this list that
three 175 MH pendants are a good combo for a 125 gallon open-top tank.  You
can even get by with two pendants.  I happen to have two Hamilton 175W
pendants, and was considering buying a third one to use if I buy the 180
gallon tank.  Hamilton's recommendations on their web page are on the
border.  Under the bulbs section, they mention 175 W MH is good up to 24
inch depth, and mention 250 W MH is good for 24 inch depths and greater.  I
don't want to be aching later for more light, but would like to reuse the
pendants I have.  They work great with the 21.5 inch depth of my 75 gallon
tank.  Any suggestions?
MH is my favorite :-)

You will be fine with 175w MH lights over either a 125 or a 180 tank, the two inch difference won't be noticeable in terms of light levels. I think 250w would be overkill and might actually be a problem level of light. You will want three pendents for either a 125g or a 180g tank, since a pendent typically is good to about a 2 x 2 foot square area -- with a 125g you get some spill over on the front/back, on a 180g things work out nicely. I think using only 2 pendents over either tank will result in shadow areas and some bizarre light angles that might be undesirable due to the length of the tank and the coverage areas of the pendents. Both the gallon capacity AND the dimensions of the tank determine the lighting you will need, watts/gallon starts to be less useful on larger tanks where other factors come into play.

Keep in mind that a lot of the depth levels given for MH lighting are given with the assumption that you're lighting marine tanks with corals. Light levels tend to be higher in those types of tanks than they are in planted freshwater tanks. Also remember that when you put in your nice, deep substrate you're going to take 3-4" off of the effective depth of the tank, so with a 180g tank you'll actually only have 20"-21" of water depth that the light will need to penetrate. With the pendents mounted above your tank maybe 12"-16" and a 3"-4" substrate you should be fine with 175w pendents.

Also, does anyone on the list own a 180 gallon tank made by All-Glass
Aquarium Company, Inc?  If so, can you measure the actual dimensions of the
outside frame to an eigth inch?  I don't know if the nominal dimensions
(72x24x24) are to the outside of the glass, or the outside of the frame.  I
always thought the manufacturer's posted dimensions were to the outside of
the glass, but Oceanic says the dimensions for my 75 gallon are
(48.5x18.5x21.5) and that's to the outside of the frame.  I need it to
design the base.

The last page of the All Glass catalog (available on their web site as a PDF file) contains "actual" dimensions of their tanks, which are to the outside edges of the plastic frame IME. I did actually measure one of their 125g tanks in a store and it was just about right on the numbers they gave in their catalog. Probably would be best to measure a real tank in a store first if you're going to be using very tight tolerances for your tank mounting location.


Thanks in advance,

Steve Pituch

Waveform Technology
UNIX Systems Administrator