Anacharis is the way to go. I've grown it with the standard fluorescents that come with tanks (only 2 watts per gallon) and you can grow it planted or just let it float on top. It does best floating when you have low light. You can also buy fake plants and just let them float on top, or buy fake tall enough to reach the top and maybe bend over a bit. That'll provide plenty of top cover. The plants on the bottom won't do much, guppy fry hang out at the top where the food is. Putting a divider in a 20 gallon should be plenty to start with. Even if you don't separate the parents you'll be overrun with fry in no time. Like rabbits...

Z

Did you know that a female can give birth to many (between 2-6) batches of fry from ONE mating? She stores the sperm of the male and as soon as she gives birth the next batch start to grow. I had a female guppy have 5 batches AFTER the male died. Also had a platy who gave birth 3 times after I gave away her male. Google search Reproduction in live bearers and you will find articles on it.

Just thought I would pass that on so you are prepared. Even after separating the males and females, and the females all give birth, chances of many more batches after that are very high. :-D

"During mating, sperm are transferred into the female’s body in the form of a sperm packet. This dissolves within about 15 minutes, freeing the sperm. Any sperm that fail to fertilize immediately do not die off, as happens in the mammalian reproductive tract, but instead remain viable for the life of the female, probably nourished by and output of sugars from the ovaries. This is why a female live bearer housed on her own can still give birth to successive broods of fry, using stored sperm from the past matings to fertilize the eggs. As a result, a number of males may be responsible for the young born in a single-brood."

you really dont need to do much, maybe pick up some food for the fry since they will be too small to eat flakes. it might be a good idea to have a seperate small tank for the babies, less a lot of them be eaten.

ive heard you can feed them baby brine shrimp with finely crushed flakes. you local pet store may also have packaged food specificly for fry

What I do is put my pregnant female in a 1 gallon tank so its bigger then a breeder box and when I think shes close I will check the tank and suck the babies out with a baster and put them in their own tank hope this helps.