The algae is not there because you've been using rain water, all water has the potential to grow algae. If the water is stagnant, this is a large factor, and you should add a small pump to circulate the water. Either that or an air pump with airline tubing and an air stone in the water. There are also algae-eating snails, but they also tend to eat all other plant matter as well. How much lighting do you have on this tank? How many plants? How many animals? How often do you change the water? How deep is the water? Is this a pond sort of area? The algae won't actually harm anything, it just looks gross, so you could just manually remove it regularly. Aquatic plants would also help a lot. Try a real nutrient hog like hornwort. If you think it's ugly (I do), get something else, like anubias, pennywort, valisneria, java moss, etc. Basically any true aquatic plant will help to use the nutrients that are in the water, and will out-compete the algae. But in order to give your plants a head start, manually remove as much of the algae as you can before adding them. And still get a pump to move the water. Plants don't really like stagnant water, either. A marginal plant might help as well, like a small peace lily or lucky bamboo growing directly in the water (but just the roots).

I would reccomend more frequent water changes. Animal waste rapidly fuels algae growth. Also, like sahoy said, any aquatic/marginal plant would help significantly. Marginals would probably be easier to plant though.

The terrarium is multi-level.. Base is 2.5' by 4' & it's 3' tall with 4 shelves. The water isn't more than 1/2 inch deep at any given time- There's probably 200 plants in there, no critters. Total wattage is about ... 250 of fluorescents. The algae's just growing on the aluminum plates which form the bottom of the shelves.

I was thinking there might be some water additive or something...

Most petstores sell algae killing solutions that "claim" not to harm plants. They sometimes work, sometimes don't, and there is a chance that they could kill/harm you plants.

I'm still confused about your setup. In your first post, you mentioned "carnivores." At first I thought these were animals, but now I realize they are plants. Are they sitting in the pool of water? Is there a pool of water on each shelf that pots sit in or something?

Exactly. Carnivourous plants like sitting in water- it replicates their natural boggy surroundings.

I'd take a picture of the thing, but with the lights on they fake out the camera & it's too dark. Without them on... It's still too dark.

Ahhh. . . I see what kind of set-up you have now. Except for one thing: if there's a tray on each shelf for holding the water, how does the light get to the lower shelves? Are they staggered?

I don't think any chemicals would be safe for use on carnivores, since they don't even like tap water. And since lots of water changes like that would get tedious, your best bet is probably aquatic plants. Even something small, like azolla or duckweed, would not only use the nutrients in the water, but it would also help to cover the water surface, blocking the light, and further reducing algae. Wouldn't harm your plants, either.

I thought of building an inverted piramid.. But instead I just put lights under each shelf.

Duckweed is an interesting idea.. I wonder if there's a pond around here somewhere I can get close enough to get some.. There's many creeks & such around here.. But I've found most aren't terribly accessible to one in a wheelchair. So the hunt will continue. Of cource I could always buy some, but I'd rather hunt my own or trade for it.

I'm sure that someone with a pond would be glad to give you some duckweed or azolla! From what I've read, the stuff multiplies like crazy. I used to have some, but regretted buying it almost right away. Tough to get rid of. However, I think that it's exactly what you need in your case. Mine was in a fish tank, and it kept getting blown around by the filter and caught on stuff. It prefers still water.