One of my goldfish has developed a dark red sore between his pelvic and anal fins. Several days ago he had a red pimple like sore on his gill plate which went away on its own. He has also been bottom sitting quite a bit in the last two or three weeks, though still eats and swims as normal and reacts normally to any type of stimulus. Other goldfish in the tank are all fine, parameters are good. Any idea what's going on? I'm leaning towards a bacterial infection, but just don't know... Any downside to putting in a divider and feeding him Jungle anti-bacterial food?

I found a site about goldfish ulcers...could he have that??

This is what it said:
Goldfish ulcers are open sores caused by any one of a range of goldfish illnesses. They are the result of infection of a wound caused by injury or parasites and brought on by poor water quality. Ulcers are usually caused by the aeromonas bacteria.

Goldfish suffering from ulcers have red or pink sores on their body, sometimes with a white centre. Sometimes other goldfish illnesses can appear at the same time, such as tail and fin rot.

Treatment of Goldfish Ulcers

When treating goldfish ulcers make sure that parasites such as anchor worm, chilodonella, costia, fish lice, goldfish ick or skin flukes are not present.

A vet may recommend a course of antibiotic injections, or remedies available from pet shops may successfully treat the goldfish ulcers. Goldfish ulcers can become infected with other bacteria or fungus.

What I would do: Take a picture or a cell phone video or something that clearly shows the red sore to the pet store and ask them to get the medication that treats that.

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10gal fry tank planted with Wisteria, Java Moss, Anuabis and Anacharis
35gal Sponge Bob tank with live bearing tropicals (daughters tank)
15gal planted with Swords, Java Fern, Wisteria and Red Rubin housing 10 fry and 2 bladder snails
55 gal with Oscar (future planted tank with angels and flowerhorns)
55 gal Community tank (platys, mollys, guppys, glass catfish, angels, phantom tetra, red eye tetra and swordtails)