Zebra Danio With Arched Back

hi .this week i noticed that one of my zebra danios was swimming around at the top of the tank with red swollen gills and an arched back. no other fish are looking sick and all my other danios are perfectly happy. i changed the water (about 30%) but the problem has persisted and now my fish has sunk to the bottom of the tank with a dying look about it. it had always had a very slightly arched look to its back but nowhere near the extent of what it is now. i dont think it will live but i want t know whats wrong so that it doesnt happen to any other fish. thanks.

Size of tank in gallons or litres.
How many fish and which type.
Water stats in ammonia, nitrite, nitrate, and ph.

How old is the fish as danio backs can bend with old age.

Bent spine can mean old age, injury, fish tb, internal parasites, fish tb, vitamin deficiency.
Red gills can be bad water quality, bacterial gill rot, gill flukes.

Do the gills look inflamed or look like there bleeding.
Any excess mucas on the gills.
Any signs of flicking and rubbing, laboured breathing, darting, gasping, or erratic swimming.

He dson't sound good it might be best to end his misery.

110l tank, 5 zebras, 5 rummys, 5 silver tips, 5 leaopard corries. ammonia 0, nitrite 0, nitrate <25. i water change regularly. the gills are just largely inflamed and red. it is about 1yr old. the gills dont appear to be bleeding or have excess mucus. It is breathing erratically but as for darting and erratic swimming, it is lying on its side in the gravel. How would you reccmend "ending his misery"
thanks alot.

wait a minute! it appears to have risen from the gravel but it still doesnt look good
i got a picture!!!

Get a jug of very cold water and add ice cubes till the water freezing cold. If no ice cubes just put the jug of water in the freezer till a thin sheet of ice appears on the top.
Place the fish in the freezing water. it won't take long.
Make sure gills are not moving and leave the fish a while to make sure its passed away.