I got my daughter a 55g tank for x-mas, it was an upgade from a 20g. We added about 7 fish over a couple weeks then suddenly notices they were dying one after another. We lost a huge angel, a huge goldfish that was already under turmoil and she was from old tank, a black goldfish from old tank, a one eyed black bug eyed fish that was new, a sucky fish was discovered while emptying tank (he was very blue in color I amsuming he was dead a while), and a tiny angel just dissapeared. I did a 75% water change and waited about a wk before putting decorations back in tank. A wk later (today) I did a 50% water change and put in new active carbon filters and also added a under gravel filter system without placing the carbon in as I want as much water movement as possible and have been told that those small filters only slow the flow. I am reluctant to add chemicals to the water so the only thing I have put in it is the dechlorinater. I have not lost any more fish since I did the 75% replacement, yes I had survivors. 4 goldfish, some kind of pinkish fish, a large algae eater, These survivors were all from the old tank. The newly added that made it include a small albino algae eater and about 7 tiny fish with fluorescent markings. Could it be possible that there were too many fish and not enough water changes. I have no clue what went wrong and I am heartbroken over the ones that died , of course they were all the unique ones. I am here to learn so I can improve my fishies lives, I hope you all can help with great advice.