Snakes typically represent lies or evil people in dreams. The color black represents unrighteousness and evil. White is the color of purity and that is why God children will be given white robes in heaven.
The rain represents an outpouring of spiritual blessings, unless the rain water is black or dirty. Since you did not mention any strange color was associated with the rain color I assume it was normal water.
The Bible talks about the fact that the latter rain will be greater than the former rain, and the former rain represents the days when the apostles walked around and healed many people and did many miracles in the name of Jesus Christ.
A fish represents a follower of Christ. You sometimes see Christians placing a fish symbol on their cars.
The color red is the color of the blood of Jesus. This means a red fish represents a born again Christian who has been cleansed by the blood of Jesus - which came about through the rain which represents a down pouring or falling of the Holy Spirit.
So the evil person - under your careful leadership and direction - found his way to the rain and salvation through the blood of Jesus Christ.
This dream seems to be a vision of God's calling on your life, that in the days to come, when the days of judgment come and He pours our his Holy Spirit for the last great harvest, that you may be there to lead someone to the Lord.
I don't know if any of this makes sense to you, but I hope in my heart this interpretation is true and you have a wonderful call on your life.