Fish sickness?

You must have made a mistake.

sounds like red pest http://www.aquariumlife.net/articles/fish-diseases...

or ammonia poisoning from not changing the water often enough. Once they are poisoned, rapid water changes rarely save them.

Sounds like some sort of poisoning.

The most likely culprit is that you didn't add dechlorinator to the water that removes chlorine/chloramine, or used an outdated method of getting rid of chlorine such as letting the water sit for a few days or boiling the water before use that would work for chlorine but wouldn't remove the chloramine that most tap water companies have switched to.

Another possibility is poisoning from chemicals or soap residue. If you used buckets that had been exposed to soap, exposed your fish equipment to soap, actually cleaned something with soap, or had some soap suds in the sink as you handled the fish buckets that all could cause soap poisoning for the fish.

I also had the same thing you are experiencing happen to me when my tap water company switched from chlorine to chloramine without informing me. I had a dechlorinator to neutralize both, but chloramine is an ammonia-chlorine compound which caused some free Ammonia to be present in my tap water. My hardy fish were fine, but in the short time it took (a day or two) for the beneficial bacteria in my cycled tank to remove the Ammonia from the tap water I had a mass die off of my sensitive species. My water tested fine by the time I thought to test it, but I lost even more fish when I changed the water again, and when I tested the tap water itself I found it had 1 ppm Ammonia in it. I then had to switch my dechlorinator to Prime, which detoxifies Ammonia long enough for my cycled tank to remove it from the influx of my tap water.

Ok. How big the tank and how Much water did you change-Never change more than 1/3. Did you clean Under the gravel? And Only use water that has 'settled' overnight-NEVER straight from the tap. Did you Replace the filters or just rinse it off? It should be rinsed and put back in-keep doing that till it falls apart. The red streaks can be from a variety of issues, from a infection to them rubbing themselves against the decorations in the tank. You might try a broad spectrum antibiotic in the tank(take out the carbon filters when treating).

The filtration system: Is it big enough for the tank? Always go one step further with filtration-if a 20 gallon tank, use a filter system for a 30 gallon tank.

NEVER clean parts of the tank with SOAP-even a slight soap residue will kill fish.

Did you treat the water you used to do the water change for both chlorine and chloramines? Unlike chlorine which will dissipate from tap water after 24 hours chloramines don't and must be neutralized using a chemical neutralizer. You can call your water provider and ask whether they add chloramines to the tap water to confirm if this was the cause.