white spot/string on calico goldfish

Question
Tank has been set up for about 4 months, but just added this fish yesterday. Tank is a 10 gallon. I believe it is a fancy calico goldfish. 2 other fish. Filter is a Tetra whisper. Checked levels today all are well, ph was 6.2. Partial water change about 1-2 gallons every week and a half or so.   Purchased from Petco (2nd attempt to add another fish to this tank, last one didnt make it).  Brought this fish yesterday from Petco and watched him in the store for a bit before purchasing, just to make sure.  He actually seems to be doing fine, except today I noticed a white string like "thing" on the side of his dorsal fin.  I have looked at the Anchor worm pics and it does not seem to look like that, I am thinking that it is a fungus, but looks more "long" than like a "spot".  As I said the fish seems fine and we just got him yesterday and it definitely was not there, so I am a little baffled at what it could be.  He is eating and swimming and his fins (as of today) are all up and he does not look "stressed".  Please help, I don't think my daughter can take burying another fish.  Thanks, C

Answer
Hi Cheryl;

I would suspect he is in shock from being moved to a new home and it's possible he has an injury from transport. Sometimes fish get hurt from netting or being captured and it doesn't show up until a day or so later. You didn't mention what kind of fish the others are and how long they have been in the tank but perhaps they are bullying or picking on your new one? If you have lost other fish and the originals still seem to do okay, you may have some aggressive guys in there. Goldfish can be aggressive to other goldfish too. If so, he may need a separate tank.

It's possible it's too cramped in there for him. A ten gallon is actually only large enough for one goldfish because of their potential size (6 to 8 inches) and they are so messy. Goldfish, even fancy varieties, are cold water fish too. If you have a heater in there or the tank water is higher than about 75f he may be stressed from that. Here is a web page about goldfish and their care;

http://www.firsttankguide.net/goldfish.php

At Your Service;
Chris Robbins