You need to do more water changes asap! High ammonia and nitrites and otherwise poor water quality will cause the veins in the tail to expand, causing the tail to look pink. A test kit is very important, but I can assure you, you need to change the water again now -- at least 2/3 of the tank while vacuuming the gravel. Don't touch the filter at all unless you want to slosh the media in your old tank water to remove the big stuff off of it. Use dechlorinator and add water that closely matches your tank temp. Cut back on feeding until your tank stabilises and consider getting at least a 20 gallon tank -- a 10 isn't enough space for 2 goldfish.
__________________ Tanks everywhere, a well-stocked garden pond outside and a Horde of Flatcoated Retrievers  |