We've also discovered that all HTML is not the same.  When I copy from MS Word and paste it in here, the punctuation turns to a nonsense series of symbols. <sigh> I guess a little proofing is always important.

Last February I brought some plants home from a North Carolina pond. (In northeastern Illinois, just the idea of ambling around in a pond in February is pretty neat.)

I put them into a fairly well lit wide-mouthed gallon jar until I could get to them. Some of the plants died. Bacopa (there is a Bacopa carolina) Val and a larger (macro) algae, among others, did quite well. I think that the algae was Chara, which has also appeared one summer in the Daphnia cultures "out back" after probably being blown in as spores.

I gave the Chara to a teaching friend and it became a fixture in the class room rather than getting returned. Following your approach of wondering what would appear, I didn't rinse them very thoroughly when getting ready to bring them home. They had had most of their water drained and were tied in a deflated plastic bag in my luggage along with several other plants for the flight home.

The jar has had 50% partial water changes, when i was doing the fish tanks. When evaporation dropped their water level, R.O. water (without hardly any mineral) was used to top off the jar.

To my surprise and delight, more Chara showed up there this summer. Not only that, but a small bladderwort also "appeared." Bladderwort includes a couple hundred species of aquatic carnivorous plants. They feed on animal material through entrapping tiny critters such as protists and even tiny Daphnia in their bladders. One can see the bladders in this image:
http://commons.wikimedia.org/wiki/Image:Uk_pond_bladderwort2.jpg

Except that they might take too many microscopic (potentially fry food) animals from a guppy tank, I don't think that bladderworts are a threat to baby guppies. Now with tiny baby rainbowfish, gouramis, Bettas and lampeyes, that might be a different story!

Here's  bit more on them:
http://www.botany.org/Carnivorous_Plants/Utricularia.php

Obviously both plant fragments and spores were carried on the other plants and in a tiny bit of water into that jar. So if time and tank (jar) space are available, one could make a case for not
trying to treat the water for hitch-hikers, at least until most possible plant candidates have grown up.

I mentioned Cyclops before. They "appeared" in the outdoor Daphnia cultures, probably brought in from the remnant of a nearby wetland on whiskers of the ever intrusive raccoons. The varmints probably also were responsible for the sudden appearance of seed shrimp or Ostrocods one spring. While the Ostrocods haven't lasted long enough indoors to make inroads into any algae, they tend to bloom in the spring not long after some of than annoying bush algae starts to grow. A certain number of the veggie eating seed shrimp remain, mostly munching decaying leaves. The bush algae disappears until the next spring.

For images of Chara please check out:
http://biology.unm.edu/ccouncil/Biology_203/Images/Non-floweringPlants/chara.jpg
http://www.scientificillustrator.com/illustration/botanical/chara.html