I'm planning a low-maintenance 90 gal planted tank. The dimensions are 48" x 18" x h=24". What would be the right lighting for this setup? HOw many watts PC? Can I add CO2 injection and Fertilizers for the plants? I just want to minimize water changes to once a month due to a busy schedule.

If you are truly looking to have once-per-month water changes, I would suggest that you consider adding very little in the way of fertilizers (Tom may have some more to offer here?!?). CO2 is beneficial for almost any tank and would be for yours, even if you are striving for a low-maintenance set-up.

For PC lighting, you could easily set up a very workable system with two of the 35" long 96W PC lights, slightly staggered from each other. An alternative you may wish to consider, is going with a 48" wide two light T5 set-up, utilizing two 54-watt T5 lamps with individual parabolic reflectors... don't be folloed by the fact that this would only be 108 watts of lighting. T5 lamps with the individual parabolic reflectors pack a heckuva wallop. This lighting, combined with CO2 injection and very moderate addition of fertilizers (maybe only 1x per week) is plenty to provide the foundation for a very nice system!

-Jimbob

I'd go 4 x 40 watt(all triton bulbs) or 2 x T5 54w(more open top design).

This will keep the tank slower growing.
If you have a truly busy schedule, you may want to opt for automatic water changing.

Many use a solenoid and plumb the tap water through a carbon cartiage filter prior to being added to the tank. The other thing is the water is replaced and removed slowly.

1/4" in, 3/8" out solenoid drain in the sump.
The 3/8" line just goes outside or to the house drain(needs to be lower than the tank's water level.

You use a float switch to shut the water off for the fill line and the solenoid is connected to timer.

So you drain and fill at the same time for say 1 hour etc, or 2 hours etc each day, 2 x week etc.

I think while it takes some time to set up, this will work very well for folks with busy schedules.

Then all you do is add ferts and fish food, trim once in a while(use slower growing plants for that).

You can go non CO2, see the article here.
That would mean less than once a month water changes, virtually no water changes(once every 3-6 months), just top offs.

Low light CO2 produces nice growth. A good fish load and good substrate will go a long way if you want to go this route.

Then dosing 2x a week light should do it.
Plant choice will be critical.

Lots of driftwood and java fern(narrow leaf), Crypts, moss etc

White sand front foreground is easy to deal with also.

Regards,
Tom Barr