Brown stomach on goldfish?

Question
Hello, I have a baby goldfish who has been with me for about 1-2 months
now.

He has been healthy lately, no weird behaviour at all.
I have him in a 5gal tank alone since his friend died.
I change the water regularly , 25-50% every 3 days and 75% every week.

The weird thing is, i notice he has a brown coloured smudge under his
tummy, up of his poo hole. At first I thought it is just a reflection of the
pebbles, but now I am sure it is ON his skin. I am very worried. Do you know
what is going on with him? He is very much healthy in a behaviour way. Eat
normally etc. No swim bladder. Clean water. But one thing also, I rarely see he
poops lately. I try feeding him thawed baby peas, but he wont eat it. I am very
worried.

What is that brown smudge under his tummy?

Thanks a lot,

Irene

Answer
Hi Irene,

I apologize for the slow response; I received quite a load of questions!

Since I don't have a picture, I can't be 100% sure, but brown spots are usually a sign of bad water quality. Try changing 20% of the water every day, for a week. This will remove any toxins, and prevent an infection.

If after a week,and it doesn't improve, or if the fish gets worse, then go to your local fish store and buy an antibacterial medicine, such as Maracyn. Follow the directions carefully, and remember to remove any carbon that may be in your filter.

When you say 'no swim bladder', do you mean no swim bladder disease? All fish have a swim bladder, or else they'd all stuck on he ground, but not all fish have swim bladder disease. Just thought I would point that out.

Good Luck, and I hope your fish recover!