We currently have one yellow common goldfish named Becky Lightening. We're keeping her in a 10 gallon tank as of now, even though I know we should get a bigger tank. (How big do you think the tank should be for one common goldfish?) We previously had her and 2 other common goldfish in a pond, but we've had to remove the pond because of it leaking water into our basement. The two other fish who were with her in the tank died. Recently, the edges of her tail have turned black and started to rot, her fins have gotten red streaks in them, and she has asmall red sore on her body. She also tends to "clench" her top fin to her body instead of letting it fan out. I've noticed her darting around the tank as well, sometimes it's when we walk by the tank. When we clean the poop out of the tank, we've found scales that must've come off of her. We recently got a bigger filter, one made for a 20 gallon tank to make up for the small size and to put more oxygen into the tank. However, She's been going to the top corner of the tank and looks like she's gasping for breath even though we got a larger filter. We currently are doing 2 25% water changes every week (Sunday and Wednesday), along with cleaning the poop out of the tank at least once a day and feeding her pellet food twice a day (6-7 pieces). We condition the tap water with Aqua Safe Plus before we put it in the tank, make sure it's not too hot/cold, and we've recently started adding Prime to the new water to remove chlorine/chloramine/ammonia. We then pour the water in the tank. In order to help get rid of thesore and redness in her tail, we used the API Furan-2 to treat bacterial fish disease. (We have seen some improvement in the redness of her tail and the sore has gotten smaller). We have also used the API pH Down in the past to lower the pH as it's pretty high, although it didn't work very well. We use the Jungle 5-N-1 Test Strips to test the water, we try to test the water once a week or once every 2 weeks, and the water parameters are: Nitrate: Safe (around 10-20) Nitrite: Safe (0) Total Hardness: Very soft (around 0-25) Total Alkalinity: High (300) pH: Alkaline (8.4) If you could please explain why she is getting sick so badly, I'd appreciate it. Also, if you could give any product recommendations for some of our water issues that would be great. Any other tips or critiques to our care of Becky and her tank would be so helpful. It might help to know that when we first got the tank (almost 2 years ago), we didn't properly cycle the water and just followed the instructions that came with our tank. (Bad, I know, but we didn't know any better). I don't want to have to relocate her because I'm not sure where we would take her, so I hope we can fix these issues. Thank you (The water may look a little cloudy/yellow because of the API Furan-2 treatment we recently put in).