Negative:
Dreaming of an empty tank could represent a lack of wisdom to wins souls. Some fish grow according to the environment or tank they are in, therefore, if the tank is too small the growth of the fish will be stunted and its lifespan shortened. To dream of a small tank could represent stagnation, being out of the will of God, or spiritual malnourishment and death.
Wisdom: If you dream of a an empty fish tank, pray that God gives you more wisdom to win souls, making you a victorious fisher of men (Proverbs 11:30). It is also important to study the Word of God because it is essential to your spiritual growth. God has need of you to do what matters to win souls and reap the harvest. If you have become stagnant and lost your salt (savor) repent and return to your first love. Pray that the zeal of the Lord consumes you to fulfill the will of God!