Gone for 2 days huge problems when i came back

I went away on a trip for 2 days left fish in the care of parents who r used to doing this, i came back to grim ph lvls plummeting from 7.2 - 6.0, 4 fish are dead i replaced some of the water to make up for the sudden drop, ammonia lvls are normal, also some of the fish have their fins covered in bubbles actually most of them do any idea what might be causing this ( the bubbles ) or the ph drop, nothing new was done to the tank over my time out also i found one of the fish still alive half dead gasping for air i put him into the other tank where the ph lvls are normal and the other fish are fine any tips on treatment to try and keep him alive would be great