Fish nearly died, now abnormal behaviour

[avz10]

It is summer in South Africa and my fish are(were) very healthy. I was away for the week, and on Thursday my daughter phoned, saying that three of the fish are dying, gasping for air, having "convulsions", sinking to the bottom, balance gonee, and turned upside down.

I told her to backwash the water and add new water. While doing so, she realised that the Venturi was not working and that their could have been an oxygen deficit. (The water level in the sump was too low).That was rectified.

A massive water change was done, etc, etc. One fish recovered totally. Two others are acting very strange/sick, but have been surviving up to now. This is Saturday afternoon. Testing the water is of no use now, have changed a lot of the water. All the other fish are fine.

I spoke to my koi guy, but he had no other explanations, except for a big pH drop or swimming bladder.

Will try and post a video