If someone washed a dog in a bath tub and this bath tub was cleaned using bleach and a sponge then this same sponge was used to clean another shower, is this second shower classed as najis?
Short answer No
It is not najis because the sponge is clean(not najis). Just because sponge was used to clean the "bathtub"(where the dog was washed) it doesn't mean that the sponge is unclean(najis). And you used bleach, so far I know that bleach is used to clean the unclean surface. And the sponge was in contact with the bleach. So the sponge is also clean(not najis).