fish do best in a habitat with:
A. Shelter
B. enrichment
C. space.
This stroller nails...well, none of that.
If the goldfish aren´t in it for long, and have a main habitat with all those things, it might be fine.
However, being in the stroller causes stress, which can, in turn, lead to disease.
Anyways, you just had a lesson from me, who is a self proclaimed fish and aquarium expert.
Hope you understand this! :D