My goldfish keeps floating to the top of the tank. He struggles to swim down but within seconds has floated straight back up again and just sits still at top of water. Whats wrong?

Sounds exactly like swim bladder infection. In many cases your fish will die.

Usually this infection occurs due to lack of salt in the water or poor water quality. Even though goldfish are freshwater fish, they need the addition of 1/4 cup salt per 10 gallons of water. I would do a partial water change of about 1/4 of the water today. Wait a day and do it again. Add half of the salt with this partial water change, and the rest in the next partial water change tomorrow.

The only other hope you have is to find a veterinarian or pet shop who would be willing to administer tetracycline injections. Most will not do this. Adding antibiotics to the water will NOT WORK. Don't let anyone sell you antibiotics to add to the water.

Unfortunately, your goldfish may live for quite some time with this condition. If left untreated, it is a slow death. If you don't plan to pursue the tetracycline injections, and you don't see results within 30 says of the partial water changes and the addition of salt, it would be best to put the fish out of it's misery.

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If the water quality is extremely poor, you will need to continue to do more partial water changes after the first two. But I would space those out about 3 to 4 days. You don't want to put the fish in shock also. And don't forget that with each partial water change, you will need to replenish the salt accordingly.

One common complaint I used to hear at the pet store was, "My fish was fine a couple of days ago." Even though it may appear that way, the water quality is gradually declining if you have not done partial water changes once a month. Additionally, if the tank is evaporating, and all you are doing is topping it off with fresh water, this is not considered freshening the water. The water will evaporate pure, leaving behind all minerals and trace elements. Thus leading to a hardening of the water. This is also why sometimes, you may add a "new fish" to a tank that appears to be fine, and the new fish will die. The current fish have been slowly acclimating to the declining water quality.