Hiya gf
Ive done a bit of research into MH lighting in the past for a project i was considering at one time, anyway i came down to PCs or T5s in the end but thats not to say MHs are'nt for you.
Anyway heres what i came up with based on some notes i had written at the time so just keep in mind i do not and never have owned MH lighting this is just based on research i did about 3 months ago, also i never got as far as checking out individual types of bulbs so i dont really have any info on that.
Anyway here what i came up with, all related to planted tanks and MH light fixtures>>>>>
I dont think the WPG rule is really any different for MH lighting to PCs or T5s the difference really is that the light from MH is not diffused as light would be from PCs or T5s, it is more direct and therefore is more intense, this intensity is what gives the ripple effect on the substrate of planted tanks.
This intensity can effect your plants when you switch over to MH from flourescents, also MH bulbs give out a lot of UV, i dont think this is really a problem but the initial changeover to more intense light with UV can shock your plants and you can get a burning effect on the older leaves but the new growth adjusts over time.
MH bulbs need about 100 hours running in time before the colour in the bulb balances out to what it should be, they need a burn in period, also bulbs even in the 6000 to 6500k range can look a bit yellow so just be aware of this when you go to pick a bulb type as they are very expensive to change if you dont like the colour of the bulb, i think some people will run a bulb from the bluer end of the spectrum to balance out the yellow (but i never got this far into looking at individual type bulbs)
The bulbs need to be replaced on an annual basis, the bulbs undergo a spectral shift as they age, some people even suggested changeing them as frequently as every 10 months, and obviously the costs can mount up quickly as the bulbs are very expensive to begin with.
The bulbs have to cool down for about 20 minutes before you can do any maintainence, if they are not cooled down and you mess with them they will blow.
Another thing to give serious consideration to is condensation on the walls of your house if useing MH fixtures and an open top tank.
So for your own proposed setup with 2x250 MH bulbs you would be running 4.16 WPG with no ability to reduce the lighting other than adjusting the light fixture up or down above the tank, another option is 2x175 MH bulbs and this would give you just over 2.9 WPG, i dont think they do 200 watt MH bulbs although i may be wrong.
Anyway thats my input based on research done a while ago, id say the main thing to consider is the condensation and the particular type of bulb, make sure you like the colour, and also the running costs, although i dont think its considerably more expensive than running the equivelent wattage with other fixtures.
Also would you consider a 48x24x20D, and why do you want a 24 deep tank, just curious to know, IMO the 48x24x24 can look a bit cube like for a planted tank, i think 48x24x20 can look better especially with an open top (i think Amanos tanks in that size range are 48x24x20D but id have to double check, and its not the reason i would go with that size, but to me it looks better) but thats just personal taste i suppose at the end of the day, and you can buy a 48x24x24 off the shelf.
Keep us informed of any developments, i knew Digweed would have you splashing out loads of cash, but a big planted tank in that size range would be really cool whatever you decide.
