Fish sick. Need advice

[cmjb13]

Have a 55 gallon tank. It's been in use for a few years. I recently had two fish die, leaving a pleco & goldfish. We waited a few weeks before going to Petsmart to get another goldfish. The first night I noticed what looked like shreaded fins. I thought one of the bigger fish may have eaten it. Upon closer inspection, there were red hairline worms everywhere, presumedly after pooping.

I quarantined the new fish (lesson learned, I should have done this beforehand) in a bucket and noticed small worms in the tank the next morning. I have since returned the fish, however the tank still has the worms, preseumedly both fish are infected. I went to the local mom and pop fish store and they recommended prazi-pro. I've been using it for at least 2 weeks with no difference. I stopped using it a few days ago and I noticed that the gold fish had red on his fins I'm guessing because I stopped the medicine. I've changed the water several times, with no difference.

I'm desparate here. Can anyone help?

I posted a video on youtube.

fish - YouTube

You can see them squirming around in the tank. It looks like poop, but it's not.

 

[20cc]

those look like Detritus worms..happens when you overfeed your fish. i doubt thats the cause of your fish being unhealthy

 

[20cc]

check your water parameters...ammonia,nitrite,nitrate.. could be some kind of poisoning from one of those .

http://fish.mongabay.com/diseases.htm

 

[Cactuspixie]

What kind of goldfish do you have, and what kind was the one you took back?

The worms are definitely detritus worms. Fish usually love to eat them. You are offering more food than is being eaten.

How big is your pleco?

Do you thoroughly vacuum the substrate when you do your weekly water change?
Do your test results show zero ammonia, zero nitrites and 0-20ppm nitrAtes?