pests? or water?

Question
hi!
I recently set up a 15 gallon tank with plants. I started with 3 swordtails, 4 mollies, 2 albino catfish, 2 platies and 2 suckers. Last week i did the first partial water change of about 20%, the day after i noticed 2 swordtails bloated, one of them died the day after. The other is still in the quarantine tank still alive and more or less better. is it because of the water that the fish died suddenly? Yesterday I noticed my 2 albino acting very weird, they seemed to smash on things, swim upside down, float without any movement, and resting at the bottom all the time. this morning i removed both of them, put them in a shallow tank, i noticed one of them had a red patch near the left gill and  fins were a bit rotted. It died late this morning! what was the cause? Apart from that i've been noticing very small white thread like worms swimming around but often crawling on the glass, what are they? mollies seem to like them too! also i have like spots swimming, often swirling around, i've seen them also on the gravel, too tiny to see them clearly, are they pests? could these creatures kill my fish?

please help me out

Thanks in advance!

Answer
Hi Raycine! I found your question in the general Question Pool. Sorry if it's taken a while to receive a reply.

I'll answer your questions in reverse order. First off, the white things you see are flatworms, or planaria. Harmless to fish - in fact, some fish will eat them - but indicative of a problem, you are probably overfeeding or your tank is otherwise dirty. Not a 100% given, but more often than not that's what it means when you have planaria. Vacuum the gravel and cut back on food. The white wrigglers could also be mosquito larvae breeding, but this is far less likely because you probably have a filter agitating the surface of the water. Mosquitoes breed in still, stagnant water.

Now, about your fish dying. You put quite a bit of fish into a tank that you said yourself was new. When you have a new tank, you need to take it s...l...o...w. Type new tank syndrome into google.com and you'll see why. That's what you have going on, you have an immature tank with beneficial bacteria not present at the levels they should be, to process the waste that your fish are producing. Read this article to understand the nitrogen cycle, which is what having beneficial bacteria in your tank is all about:
http://freshaquarium.about.com/cs/biologicalcycle/a/nitrogencycle.htm

Once you get a handle on what those levels mean, you need to test your newish tank for three things - ammonia (must be 0 ppm - very toxic!) nitrite (also must be 0 ppm - toxic!) and nitrates (keep it below 20 ppm, if possible) and make sure that your levels match these. You have a heavy bioload with the original configuration of fish, especially the 2 suckers if these are plecostomus. See a picture of a pleco here:
http://en.wikipedia.org/wiki/Plecostomus
These fish get BIG! And produce lots of waste. You don't need a single one in your little tank, much less 2. Even if your "suckers" are Chinese Algae Eaters (see link below)
http://badmanstropicalfish.com/profiles/profile47.html
These aren't fish you want, either. Instead, choose otocinclus catfish to control algae, they look like this:
http://www.google.com/url?q=http://www.otocinclus.com/intro.html&sa=X&oi=smap&re...
These tiny fish are docile and keep algae in check.

Except for the "suckers" and albino corydoras (this is what I believe you have, you can use Google.com for an ID) your other fish are livebearers, and may interbreed. Mollies are delicate fish in freshwater, salt keeps them healthier. Corydoras don't like salt, and none of the sucking fish I figure you have like it either. So it may be wise for you to remove these fish and keep a livebearer tank with no bottom feeders (except for maybe some shrimp like cherry or ghost shrimp) feed just a little and real carefully, so that none of the food falls to the bottom, and add salt to your tank on a regular basis. Marine salt mix is best, but that needs to be measured using a device called a hydrometer. The marine salt mix has all sorts of benefits - it's made of carbonate salts that add hardness and buffering to the tank, ideal conditions for livebearers. However, it's more expensive and more of a chore than adding 1 teaspoon per 5 gallons of aquarium salt (which is the same as kosher, sea salt or non-iodized salt from the grocery store) dissolved in warm water, to the tank. This will harden your fish up a bit and heal them if they've been exposed to any kind of ammonia or nitrite, which sounds like they have - from the symptoms you describe and the newness of your tank.

I hope that helps, feel free to write back. Keeping fish is hard unless you read lots beforehand, it's really a lot more complicated than keeping a pet like a cat or dog! So here's three sites I recommend you use for researching:
http://www.wetwebmedia.com
http://freshaquarium.about.com
http://www.badmanstropicalfish.com

Take care,
Nicole Putnam