Goldfish tank mate

Question
I have had two fancy goldfish since they were babies.  They were each about 18 months old and 7 inches long.  One week ago, one of the two became sick.  The vet determined she was swollen with fluid due to either liver or kidney failure.  There was nothing they could do for my fish so I had her humanely euthanized.

Since then my remaining goldfish, who is a male, has lost his appetite and keeps swimming back and forth across his 30 gallon tank.  He seems to also keep bumping into the sides of the tank with his head (I think he sees his reflection and thinks its his deceased tank mate)

I don't know whether or not to get him another tank mate.  He' pretty big, & I'm afraid he may not take to a smaller fish.  Also, its VERY difficult to determine the sex of a fish, particularly at a young age.

Is he showing signs of needing a companion?  Do I need to quaratee a new fish & for how long?  

Answer
Hi Susan;

Oh, that's too bad. I'm sorry you lost her. Your vet is quite right. It usually isn't treatable so euthanasia is often the best thing to do.

I would wait another week and see how the surviving fish is acting. I hate to say it but it is possible that he could be sick too. Bumping into things is one of the symptoms of diseases like organ damage. The toxins in their blood from failed organs can mess with their minds. Make a 25% water change every 2 or 3 days to help boost his immune system and see how it goes.

If he is fine, you could search around for a larger goldfish roommate for him. Quarantine is always best, but depending on the size of the fish it isn't always possible. Most of us don't have larger fully biologically-balanced tanks handy to put a large fish in. You could try it with a bare 10 gallon tankinstead. Put a filter on it and change 50% of the water every day. Goldfish are pretty messy dudes and will toxify their water pretty fast. The new filter can't remove the toxins yet because the tank never went through the break-in. It will remove solids so things "look" better in there though. Keep the new guy isolated for 2 weeks or so.

I hope all goes well......

At Your Service;
Chris Robbins