white slime in guppy tank

Question
I've asked around and no one knows what it is however some have said they've seen it before and removed it.

I have another fish tank, 12 gal. with 1 swordtail in it who has been looking terrible for weeks, we've had the tank for about a month now the other swordtail died after about 2 weeks. The fish has brownish yellow dents or marks on his head and down the very top of his back, also his color is extremely dull, we've done water changes, salt and now he's been on antibiotics for 3 days and no change. He eats fairly normal but recently has eaten a bit less, he swims around and acts normal. In the last week his fins have been falling off, he has lost his tail except for that thin black line. No fish disease fits these symptoms and I haven't been able to find anyone who has seen such a thing before. My husband posted this question at  petfish.net in the freshwater fish help forum, there are pictures of the fish there if you'd like to see but he looks much worse nowadays. Any clue? I'm at a loss with this one too!
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Followup To

Question -
my daughter has a 5 gallon octagon tank with one guppy that she just got a week ago. Today I noticed this white milky slimy stuff all over the fake plant, I pulled the plant out and the slimy stuff just slid into the water, I tried scooping it out with a net but I couldn't get it all. It's gross, there was about a tablespoon worth of this stuff in the net and a lot more on the bottom of the tank.
She's fed the fish every other day, there is no ammonia in the water. Do you know what this could be? I've asked on several fish forums and no one has replied. Please let me know asap. Thank you!

Answer -
Dear Shelly,
Hmmm...I really don't know what that could be either!
Usually moldy kind of material in the aquarium is food related or due to some other pollution of any sort.

This is certainly something I've never had/heard of before.

Just make sure the aquarium is cleaned at least twice a week with 30% water changes--always insuring to make the replacement water equal in temperature to that of your aquarium and that it is properly dechlorinated with a good water conditioner.

This shouldn't be a re-occuring problem but I will ask other aquarists of they have any clue to what this might be. And if I come up with anything, I'll be sure to let you know...

Best wishes and Happy fishkeeping!
Karen~

Answer
Dear Shelly,
I'm very sorry to hear about your trouble with your poor swordtail. I honestly am not 100% sure what is wrong. But I would highly suspect poor water quality to be the culprit. There could be ammonia or nitrites in the water. Losing the first swordtail is a surefire sign that something is certainly wrong.

The marks that you described COULD be natural coloration but i'm not sure. I looked through all 15 pages of the petfish "freshwater fish help" forum but could not find your post. Maybe I just missed it but seeing the fish would certainly really help. What is the subject line? maybe I just missed it.

But above all I would change 50% of his water every day making sure the replacement water is equal in temperature to that of your aquarium and is properly dechlorinated with a good water conditioner. Medications are likely not what he needs now but clean water as soon as possible. I would stop meds but perhaps leave small dosage of salt still. Sadly, ammonia can burn and literally melt off a fishes fins but black edges and melting away can be a sign of fin rot also. Both these problems require daily 50% water changes and (if you are certain your swordtail has finrot) you can try a treatment of Melafix and pimafix used together as these are reasonably safe and not so dangerous antibiotics.

Above all a water change is in order and if you could test your water for ammonia,nitrites, and nitrates that would be best.

Thank you for your patience and you have only my very best wishes!!
Karen~