Goldfish Not Eating, Gasping

[QuanleePrime]

Two-year goldfish owner, currently holding four goldfish in a 40-gallon tank. Filter is designed for a 70-gallon tank, pH is perfect, weekly water changes. However, in the last few water changes, ONE fish of the four will suddenly get sick. Symptoms were typically of tail rot, so I've treated with Maracyn and Maracyn 2. I am attempting to isolate the problem by cleaning the surfaces of decorations and cleaning equipment.

But my moor is showing slightly different signs. He is gasping for breath, lethargic, and spitting out his food. His fins are not clamped and while he is now favoring the bottom of the tank, he occasionally makes an appearance around the middle. Aeration is fine within the tank and all other fish are healthy.

Help!

 

[fishguy2727]

Do a water change.

What is the nitrate concentration?
Do you have an air pump? If not get a big one.

 

[fluttermoth]

I'm not sure adding more aeration than there already is will help tbh, although it wouldn't hurt. OP; do you have any test results for ammonia or anything else? How big are your water changes and how are you maintaining your filter?

I sgree about the water change; it usually helps.

 

[QuanleePrime]

I just changed the water and the nitrates are fine. I change about 1/3 of the water when I do.

Filter maintenance: once a month change of the lower level sponge and every three months a change of the carbon and upper level rocks.

 

[fishguy2727]

What is the number for nitrate concentration?

You don't need to ever change any biomedia.

If you are using carbon it is better to change it monthly.

What filter do you have? It sounds like an AquaClear. If so you also never need to change the sponge, just rinse it in tank water.