Maybe its a decoration in the tank itself. Happen to people I know IRL. Also it could be a painted goldfish (people have painted fish to look pretty to sell). Maybe something easy and common is the probably then something more serious? Maybe you got rocks red/pink and didn't wash it good enough? Anything and everything..
it could be from several factors. do you have rocks or wood in the tank? they could give off coloration depending on where they come from. it could also be red algae. try using algae removers you can buy from local fish stores. did you use any other chemical that might have a reddish or pinkish pigment? try to check HERE. maybe it could help you somehow.
Do not do this. You should never ever put chemicals into your tank when you don't know what the problem is. You wouldn't give a child a bunch of pills and try them til one works would you? So why do it to the tank? And personally, I have never heard of any algae that would turn the water pink. Green, yes. Not pink.
Speaking of pink things, I have an Aquaclear filter, and the sponges of course are kind of beige. Well for me, they start beige, but after a little bit (after putting in a new one), they turn pink and stay that way. They have been doing this for years, my water is fine (drip tests, I don't know the numbers, but I know the colours). No big deal, my tank is good, just figured I'd mention it since we were talking about pink things. 