I am surprised you kept 22 goldfish in a bath to start with. I assume the majority are on the smaller size? Other wise they must be greatly overstocked.
Your standard regime is terrible! :lol: But it has obviously worked for a long time :roll: . I used to have goldfish in a bowl (gasp!) and once a week I would fill a bucket with cold water (bore water so very cold) empty the fish into it then scrub out the bowl and gravel with soapy water, rinse, then fill up with more bore water and drop the fish back in. They survived that too! I guess the fish didn't know this wasn't the right thing to do - I know I certainly didn't! :lol:
Mel's suggestion of the overflow regularly is a good one and you are less likely to have the same trouble again. Although the council say the chlorine level hasn't changed, I wonder if someone flushed the water supply to your area with something else. To have so many die so fast indicates something toxic in the water to me.
Out of curiosity, was it mainly the larger fish that died or the smaller ones?