Do you really think it is soap?
The other (major) problem is ammonia. Adding that many fish will add a lot of ammonia to the water, and it can make the water foamy with bubbles that do not pop, just like soap bubbles. Test for ammonia.
I would do a big water change, either way. Soap or ammonia are both toxic to fish.
Add activated carbon to the filter. I do not know if it helps with soap, though.