Are you buying them at the petstore?
If so then:
Any kind of Moss
Water Sprite if they have it.
A marimo ball if they have any real ones and it's not just moss attached to a cork.
I highly suggest purchasing plants online if you can though as you'll have a greater selection. ;]
None of the above plants require substrate and most are low light plants but they'll really flourish under 2 watts per gallon or more of Daylight. You can simulate this using a regular sized or miniature Daylight (6,500 K(Kelvins)) CFL (swirly bulbs).
Over 2 watts per gallon can sometime promote algae growth unless you have a lot of stem plants (anacharis, hornwort, water wisteria, water sprite) so you'll want to be careful with that or try to buy some snails like ramshorns (red or blue) or nerites (olive, tracked (orange), or zebra/striped) online.
The ramshorns can float or sink at will due to a special boyancy system they have and can be lighter than nerites so they can clean your more delicate plants (whereas nerites can sometimes be too heavy) and be very entertaining but the price for that is they breed easily so you'll want to be careful not to overfeed your fish.
Another great option to combat algae are shrimp. Red Cherry shrimp, Yellow Shrimp, and anything in the neocardina family are awesome algae eating shrimp though most freshwater shrimp do well eating algae off your plants. They don't do that well at eating it off of your glass though (like snails can) so there's that to consider.
I'd start off with ghost shrimp as they're cheaper, more readily available in-stores, and can hide morea easily from your betta. ;]
Good luck! =]
Last edited by wallywestisthebest333; 02-05-2011 at 02:55 PM.