We do not want to keep to the formalities of fasting alone. The same applies to Soya cheese. One may argue whether or not it contains if it contains ingredients of animal source. However, from a spiritual point of view, do you love cheese so much that you insist on gratifying your bodily desires during the fast by finding a substitute or an alternative? The same applies to non-dairy chocolate; do you crave that brand in particular? Why not substitute it with a cup of cocoa?
As for fish, it is primarily animal food. Although it has been permitted for the weak who cannot endure the large number of fasts, it is not permitted in first class fasts.