Giant danio with mouth stuck open

A few hours ago I went to check on my fish and found one of my giant danios gulping for air at the surface. I tested the O2 levels in the water and they were good ( medium/high ), I tested ammonia and nitrite and both were 0 and pH is still steady at 7.5 . None of the other fish in the tank are behaving abnormally. I have moved the fish to a hospital tank and added bacterial and fungal treatments. The fish is staying still at the surface but keeps it's mouth wide open, there are no signs of injury or other problems with his mouth.
Does anyone know what could cause this?
Are there other treatments I should try. ( I'm in the UK so the range of medicines legally available is somewhat limited )

 

Did you do a filter change or a large water change recently?

 

Only my usual 20% weekly change this week although I did clean out the sump 10 days ago. I tested the water yesterday with every test I have and all results were normal. The fish was dead this morning but I would still like to discover the cause.
Test results were
Temp 26.2 C
Ammonia 0
Nitrite 0
Nitrate 2.5 - 5
PH 7.5
Kh 5
GH 7-8
Cholorine 0
O2 medium / high

All these results are normal for that tank.

 

The only thing I can think of: cleaning out the sump pump removed too much of the beneficial bacteria and so you had a mini ammonia spike at some point which damaged the fish’s respiratory system/gills. Luckily only one poor fish succumbed. That’s great that all your other fish are doing fine.

 

Thanks for replying. Hopefully it's just an ammonia Spike and the rest of the fish will be fine.

 

Could it be old age? It could also be flukes. Have you seen any of the fish flashing?

 

It was only 2 years old, I haven't noticed any flashing