I have 110 gallon (420 liter) tank, which contains 1 ranchu measuring 13 cm, 1 ranchu measuring 10 cm, and 1 oranda measuring 8 cm. The oranda is gasping for air, while the other fish seem fine. There is aeration in the water. What's wrong? What should I do?
First thing you should do is isolate your fish on a medic tank just in case it does have an infectious disease it could transmit, but also for you to be able to give them some treatment without affecting the rest of the population in your tank.
Secondly, observe very well. Take pictures and look for anything different than usual. White dots like sugar, inflated bladder, tumors, etc. Observation is a key factor here.
Third, based on that, an appropriate treatment could be applied.
I know you fish is an indoor fish, but all fish have similar needs. Our outdoor Koi have many of the same needs. I agree with Omar and Lila. Our pond was built incorrectly and we had to get extra and better equipment to filter the water and add new water from waterfalls as the fish grew from 9" to 20". Test the water weekly and assure that your fish have clean, fresh, aerated water, with minimal algae and well balanced Ph. Some fish are more resistant to problems that others. Your Oranda is obviously stressed. A fish should not be at the surface for extended periods unless they are eating. Your Oranda, may be sick or may be trying to get oxygen which they get out of clean, fresh, aerated water, free of algae, so you may be having a problem with your filtering. You should consult your aquarium expert for your tank as soon as possible as this could be a serious problem.