Lighting requirements for the 90g?

Thats half circle or a corner tank? I know they make glass tops for those but I bet its expensive. But thats your best bet. If you have the ability to hang a light or even if you cant I would get a 250watt metal halide pendant. The only reason I suggest you hang it is, It will keep most of the heat off the tank and you will get better light coverage. Its the most cost effective way to get the watts that you need with out having to get a light thats too wide for the tank. As you know 42" is an off size. You would either get a 36" or a 48". A 36" Compact fluorescent is at most 192W and a 48" is about 250W. Check ebay for the light. A 250 will give you just over 2.5wpg that should be sufficient to grow any plant you want. But remember if your going to try to grow certain plants you will need co2 or at least excel amongst other nutrients. There are some plants and mosses that you can keep with the light you have. If you do go with the pendant and don't plan on adding other nutrients your going to need to keep the photo period down to no more than 6-8 hours a day to keep from severe algae problems.

Check out ahsupply.com. They have a great selection of retrofit kits.

You would need 2-3 watts per gallon (180-270 minimum) for med-high light requirement plants to survive. You could go 1-2 wpg if you only kept low light plants.

The 4 x 55watt or 6 x 55 watt retrofit kits would be the options I'd suggest.

I'd avoid the metal halide simply for the heat factor.

Ahh I see. I was looking at your avatar and when you said 42" I just assumed. Oh well. You can find all sorts of good deals on ebay. If you just want something with a lot of watts for now you can get a light from aquatraders.