Amazon Swords

18" standard flourescent tubes are 15W. 30W over a 55G is even less light than my super low light plant tank, and your tank is taller too. You cannot really use your existing lighting for much plant-wise. If you are really wanting to keep plants you'll probably have to upgrade the whole unit. You might be able to gut your housing and put in an ahsupply kit like there 36W kits. Two of these would get you 72W which would probably be sufficient for low tech tank, but I wouldnt expect a huge amount of growth, but should be sufficient to keep them alive I suppose. Take a look at them here http://www.ahsupply.com/36-55w.htm
They do show a picture of a retrofitted light that is probably similar to what you have sitting over your 55 right now, looks like they had to mount the ballast on the outside though (backside).
 
If you are at all handy in woodworking you may want to consider building an inexpensive canopy and installing a 2 x 40 watt flourescent fixture. It's not hard to do if you have time to do it and you should be able to come in under $50 if your really frugal. Looking so spend closer to $100 you could get better looking wood and such. Either this way or buying retro kits would both work for what you are trying to do.
 
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