My response will be based on that I will assume there will be fish, With newly cured Portland Cement, there is a chemical residue that come off if you would do a water rinse. You can see that happening because the water will get cloudy. I assume that may not be healthy for fish. At some point the chemical may stop coming off the cement, but I honestly could not tell you how long. IMHO - I would not.

It will also raise the pH of the water and make it alkaline for a while.

tj