I saw a mention of Marsilea in a post on r.a.plants

That was me. :-)

I was able to do a bit
of research on it, but didn't come up with much good info.

Not many people seem to keep this plant. I'm not sure why.

Sources online say that it is a "low - medium" light plant, I have an 8
watt daylight floursent light on my 6 gallon tank. No substrate, no CO2.

*No* substrate? It's a bare-bottom tank? Or do you mean no special substrate,
i.e., plain gravel?

Few plants will be happy with no substrate at all.

Sources online say that it is a "low - medium" light plant, I have an 8
watt daylight floursent light on my 6 gallon tank. No substrate, no CO2.
Would marsilea would survive under those conditions?

I'm afraid I have no idea. Growing plants in tanks that small can be weird.
Often, you need a lot more watts per gallon than you would in a larger tank.

However, you'd be more likely to grow Marsilea than other ground-cover-type
plants.

Leigh

http://www.fortunecity.com/lavender/halloween/881/