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> I'm conceptualizing a new 75g (48 x 18 x 20) planted tank and puzzling a bit
> over my lighting options. I think I'll use a Flourite or similar substrate,
> supplemental CO2, and probably a cable heater. I envision a moderately
> heavily planted tank with a good-sized fish load. I expect I'll want to keep
> plant species with minimum light requirements that range from low to high.
> I'll be using a hood, not an open top configuration.

I'd go with a Power compact set up. 2 x 55 watt 5000K bulbs and 2x 6700K
bulbs. Have the 6700's ON for 4-6 hours a day. that will allow you to grow
most any plant densely. You can just have the 2x55 watt bulbs as well if you
prefer less lighting. That would do it nicely.
If you a more a DIY'er try 4 to 6 - T-8's NO FL's with a reflector.
> I guess the rule of thumb usually tossed around for lighting is 2-4 watts
> per gallon. If I split the difference and aimed for 3 watts/gallon that
> would mean 225 watts. My first question is is that a reasonable figure?

that's plenty to grow what ever you want. You will not be limited by the
> Assuming 225 watts is my target I could get close to that with 4 55w compact
> fluorescents. I could also do 2 96w CFs plus a 28w CF.
Stick with the 55's. Do split of 1-5000K and the other 6700K.
Hard to do that so much with the others and still have an even spread. Also
all the bulbs would be the same.

>Alternatively, using
> VHO fluorescents, I could use 2 110 watt bulbs. With metal halides I'm not
> sure. Two 175s would probably be too much. A single 250 watt bulb is in the
> ballpark but I don't know if a single bulb will work well for a four foot
> long tank.

They (MH's) make for a big hood if you decide on them. Remember you are the
one that's going to have to get in there often... and moving a big hot hood
is not the best of incentives to clean or maintain one's tank! Access to
your tank is a big issue.
> So, those are the options I've thought of. Have I left anything out?
> Now, the considerations. I suppose we have:
> - initial cost

PC's high.
T-8's low you can use regular old T-12 40 watt sized bulbs set up as well if
you wish.
> - cost of ownership (electricity usage, bulb life and replacement cost)

PC's very low.
T-8's low
> - heat from the bulb(s)
> - heat from the ballasts(s)

Both low.

> - availability of suitable bulbs for planted aquaria

not a problem.

> - form factor and adaptability to the hood and its dimensions

Neither a problem.

> - aesthetics, i.e. point light from a metal halide vs. diffused light from
> the fluorescents

That's a personal decision. Plants aren't picky:)

> - dimmability

VHO are about the only ones that have this (in commercial aquarium lighting)
but you can get MH/FL's that do this but it cost lots no matter what. Not a
big issue for keeping the tank though.
> Anything else you'd include? Your comments on all of these factors as they
> relate to my various options would be greatly appreciated.
> Concerning cost, I'm somewhat price sensitive but not overly so. If one
> solution differs from another by 10 or 15% I'm not going to sweat it. If one
> is twice the other, I'll give it some consideration.

You may wish to pass on the dimmer,MH's and stick with the PC and or the T-8
FL's with an electronic ballast.
> Regarding aesthetics, I have noticed in some LFS displays that the metal
> halide equipped tanks have a bit of shimmer that is appealing. That wouldn't
> be an overriding factor for me though.

You sure? After the honeymoon's over....Bulbs are 40-65$ ea also and last
the least amount of time. Can be noisy. StilI, I like em:)
> I'm intrigued by the idea of dimming because I can't imagine that it is all
> that good for fish (maybe plants, too) to have to instantaneously transition
> from darkness to full mid-day sun and vice versa. Is this of any practical
> concern? As far as I know neither CFs or MHs are presently available in
> dimmable forms. (With the exception of a particular Coralife MH unit that
> uses a proprietary or special bulb - not really interested in that one).
> Apparently, the IceCap VHO ballasts _are_ dimmable. I did read at a lighting
> industry web site, however, that ordinary bulbs used in dimmable electronic
> ballasts represent a fire hazard. I've not seen any mention in any of the
> IceCap or Niche Engineering (makers of the Solar line of dimmers) literature
> that any particular bulbs are required to take advantage of the dimming
> feature. Can anyone shed any _light_ on this for me?

Try staging your lighting. Have 2-3 etc timers for each set of lights.
If you got a 4x55 watt set up, you could turn on one set of the 5000K bulbs
for 12 hours. That would be fairly dim at about 1.5 watts a gallon. On a
separate timer have the other pair of lights, 6700K's, come on for a midday
high noon effect for about 4-6 hours. This will lower your electrical usage
as well and extend bulb life and lower heat. Another timer is much easier
and cheaper than a dimmer. Comes close to what you want

If you decide on the T-8's you could  do a similar set up. No#1 on for 12
hours No2# on for 8 hours No#3 on for 6 hours and No#4 on for 3-4 hours.

That's what I would do it anyhow:)
Tom Barr