you said it yourself that the coolant tester showed 1 ball floating. that is almost as bad as it gets next to none floating. assuming the coolant was boiling and spitting out, water has evaporated leaving a 50/50 mixture before overheating to a unknown mixture now.
it could be a 70/30 or 80/20 ethyl-glycol/water mixture depending on how much water evaporated.
the coolant tester says it's lost it's ability to cool properly so it's time to change the coolant.