58s are a PITA because of that cross brace, and they are just tall enough that 250wattHQIs are justified. But all together, I must agree that 500watts of HQI is overkill, even with just 20,000Ks or something lower in output. If you dont have a cross brace, or are interested in doing a euro-brace mod, then by all means, a 250wattHQI is a great option, maybe a couple 39wattT5s for supplimental blue/actinic depending on the bulb you choose... since a bulb that is blue enough may not be bright enough to run the tank by itself, and a bulb that is bright enough may not be blue enough for your tastes... but thats up to you.
Running dual 150wattDEs is the other option, but thats expensive.. 2x the ballasts, reflectors, bulbs, etc (which often cost just as much if not more than 250watt versions), and dual 150s may not even be bright enough.
FWIW, my best suggestion for lighting a 58g would be T5s. A 6-8 bulb tek light would make just as much, of not more output as dual 250watt halides, but for only 6-8x39watts (240-315 watts about). The available color combinations you would be able to do, and not having to sacrifice as much output when you change types of bulbs (T5 blues are almost as intense as the daylight ones, not 1/2 like with halides). The output will easily put halides to shame, and in case you think you will 'miss the shimmer', let me tell you, I was looking at my 40Bs today that have 250watt halides over them (40B's have no cross brace, I have two 40Bs lit with 250watt halides, and one with 6x39wattT5 Tek), and due to the size of the pendant (about 8"x12") in comparison to the area of the tank, I dont get any shimmer anyways. Shimmer happens more when you get shadows cast from lights that happen to be farther across the tank... a 250wattDE over a 58g isnt going to make much if any shimmer anyways. And dual 150watters would be even worse in that regard.
I would just get a Tek (the new 44s are coming out soon), OR, do a retrofit from Reefgeek.com (use the Tek 6 bulb retrofit, but swap the reflectors for the Icecap SLR's for slightly more output) if you have a canopy. It will cost you less now, and down the road to run (both in bulb replacement costs, and the bulbs last 2x what halides do), and less heat.
I browze throught the German reefing forums, and trust me, the T5s kill the halides... many euro reefers have changed over already, as they have a multitude of options already available (more Mfg's than in the US), and some lighting mfg's are considering pulling their halide only fixtures off the market, and reducing the amount of MH/T5 combo units to meet the demand of the T5 only units. One guy I talked with in Berlin swapped out his 3x250wattDE/HQI halide system for a 6x80wattT5 system, and swears he has 2x as much light now.
Here's a tank you might like to see...
Its 10' by 30"w x 28" high, lit by nothing but twin banks of 6x80wattT5s. Thats a 430g reef with SPS and clams and under 1000 watts of lighting. You couldnt dream of that with halides.
There are many other tanks like this in the Euro forums. Pretty much everyone looking at a new reef tank over there is looking at T5s.
During the hot summer that just past in California I had to jump on a 1/10 JBJ chiller to cool it. It was getting a tad bit over 81 with a 3" fan blowing in from the back. I didn't wat to mount fans on the side due to light bleeding out. Now the chiller rarely kicks on since its getting cooler.
