The last fish in the tank has died, is it better to remove do a half water change or do a full change and clean before putting more fish in?
Had this many years back when I used to have 40 ish tropical fish, one by one they snuffed it.(Age)
Would drain tank clean everything start again and add water conditioner in leave a good few weeks (3-4).
Then restart with fish and remember float the buggers for more than 20 minutes and thermostat at one end on 27C pump at other end pumping the heated thermostat water around from the back to the front of tank.
Up the temperature to a little over 30C normally gets them randy and more fish appear