I have a 3-4 week tank that had the proper amount of starting bacteria added. I'm pretty sure the tank should be at least partially cycled. I have a small shubunkin goldfish (1.5 in), and a comet. The first comet died in 3 days because of some kind of disease. I am to blame for that, because i did not read carefully, and didn't cycle the tank before I bought the fish (the shubunkin is still alive). However, what startled me is, the shubunkin started developing these white dots, and now its tail is getting all shredded on the tips. I added coppersafe, (I only added it once), and its getting worse and worse.
The most startling thing is, the pet store gave me a replacement for the comet. That new comet died in 6 hours. I don't think that is due to any disease or ammonia. The two goldfish stayed side by side for hours (seemed to like each other).
My questions are: What is causing my fish to die? How can I stop it? I really really don't want my shubunkin to die, so is there anyway I can stop it? My comet goldfish did not have a mark on it. It was pretty lively after I put it in, and then detoriated quickly. It had no white spots, no inflamed gills, nothing. I removed it from the tank today.
BY THE WAY, my pet store is PETCO, and they overstock their tanks like CRAZY. there were like some 200 0.8in comets in a 50 gallon tank. All their other fish (like shubunkins, tetras, gouramis, etc.) are all crowded into groups of 5-20 in 3-4 gallon tanks. I discarded the water that the store gave me, and used only my own water.
However, what startled me is, the shubunkin started developing these white dots, and now its tail is getting all shredded on the tips. I added coppersafe, (I only added it once), and its getting worse and worse. 
