I have one goldfish in a 30 liter tank. (Before you jump on me for the tank being too small, it was a secret Santa present given to me in a jar. I know it needs a bigger tank but I’ve spent like 300 euro on this stupid goldfish - who I secretly love - and I’m looking into rehoming it with someone who can give it more space.)

He is tail is slightly red and streaky, and he "shines" sort of red on the top of his tail/back half of his abdomen. Has been this way for a few weeks. It's hard to get a photo of. He's still eating, and depending on the time of day has a lot of energy and is swimming around like a psycho. I've noticed, though, that he has been sitting on the bottom a lot more, which he normally doesn't do. For multiple hours a day.

This is the tank: https://olacuario.es/acuario-kit-fluva ... 2meQ8OEAQYASABEgI31fD_BwE

I’ve done a ton of tests on my tank over the past two weeks, all levels are normal, including the test for ammonia which I bought because I thought maybe he had ammonia burns. Nope. No ammonia.

Water is all properly treated. Nothing new about the tank.

Water temp can creep up to close to 80 in the afternoon, it’s hot here, so I’ve been doing 1/3 to 1/2 water changes when it gets toasty (every couple of days) and I’ve also been putting large ice cubes in the tank. My house is hot so there’s not a ton I can do.

I'm thinking he has some sort of internal infection? What should I do?

Can you post actual test results, please? Also, are you using liquid tests or paper test strips?