Hello! We're very new fish owners. My mother-in-law showed up unexpectedly with a 3" goldfish and a small bowl back in Nov 2015. Long story short -- we've made lots of rookie mistakes. Murphy (named after Murphys Law) has survived all of our mis-guided attempts to care for him (oxygen deprivation from being in a bowl, ammonia posing from being in too small a tank, etc). We're trying very hard to get this RIGHT and give him a good life and we need some help. Murphy is currently in 5 gallon tank (I know, too small!!), which at the moment has balanced levels (7.4pH, 0ppm ammonia, 0ppm nitrite, 0ppm nitrate) and we're doing daily 25-50% water changes (with tap water treated with conditioner) to keep him happy. We just purchased a 30x12 20 gallon tank, and have it set-up/running with gravel, new decorations, treated water, and a Aqueon filter. We *think* Murphy has fin rot (or it could have just been from the high-ammonia levels), but he's lost most of his upper tail. We bought some medication recommended by the pet store. My questions are... Should I treat for fin rot in his current tank, since it has balanced levels and I have to remove the filter to treat? Or would we be better off treating in the new/larger tank? We'd love to move him to his new tank as soon as possible, but want to avoid the ammonia/nitrite spikes that got him sick to begin with. I read that simply placing some of the decorations + cartridge from his current tank in the new tank will quickly establish the good bacteria and cycle the tank. Is that true? If so, do I just "float" the old cartridge in the new tank water (it won't fit in the new pump -- different size cartridge). And for how long? Do I use the old tank water as well (there's only 5 gallons)? And advice on how to help him make the move, and live comfortably in the new tank would be most appreciated! Thanks in advance for any and all help!