The aquarium does have a filter with sponge. The column you are seeing there is attached to a motor pump which pumps and recirculates the water. There is a sponge to absorb the waste collected when the water is recirculated. And there are lights too and a heater. I read it on one of the threads to turn off the lights while the fish recovers.All I see is a powerhead with just a long tube to keep the water well circulated. If he had an aqua clear filter it would have been a different story.
Meanwhile I have used API Stress Coat + each time i did the water change. When there were not much replies here I had to visit a local petshop and he advised me to use paracidol and metrogyl instead of tetracycline. Anyways fingers crossed and I hope that the fish bounces back to normal.