I use giant duckweed in my 7.5 G betta tank. From craft store (hobby lobby) I got a sheet of thin black foam and cut four circles out if it. Then I cut the middle out of the circles (fold the circles in half and cut about 1/8" or 3/16" from the edge) leaving rings. I put the duckweed in the rings and they float around with the current from the small filter I use so they do not just sit in one place and block all the light. Some of the duckweed will escape the rings so occasionally I remove the excess from in and out of the rings and put it into another aquarium. A hair pick or flea comb makes it easy to remove the excess.