Is it appropriate to place fish in the water of a Baptismal font?
The placing of fish in a baptismal is font is totally inappropriate. The baptismal font, like the entire baptistry, is "reserved for the Sacrament of Baptism and [should] be clearly worthy to serve as the place for Christians to be reborn of water and the Holy Spirit" (Christian Initiation, General Introduction, no. 25). Of all the liturgical books, the Book of Blessings, no. 1085, provides the most comprehensive description of a baptismal font:
The baptismal font, particularly one in a baptistery, should be stationary, gracefully constructed out of suitable material, of splendid beauty and spotless cleanliness; it should permit baptism by immersion, whenever this is the usage. (cf. Christian Initiation, General Introduction, no. 25). In order to enhance its force as a sign, the font should be designed in such a way that it functions as a fountain of running water; where the climate requires, provision should be made for heating the water (cf. ibid., no. 20).
The Second Vatican Council's Constitution on the Sacred Liturgy itself directs that the baptistery must be practical and dignified (Sacrosanctum Concilium, no. 128), and the Ceremonial of Bishops (no. 995) echoes this legislation. It is for the diocesan bishop to regulate the orderly and effective celebration of the liturgy in each diocese, including for the celebration of the Sacrament of Baptism.