I have made an aquarium with some youtube help then rendered to see if it's Ok (and it's not).
My video to check: https://www.youtube.com/watch?v=J6HydYXtbdk&feature=youtu.be
You can see on the video that it's flickering (like the bottom of the aquarium). I don't know why it's happening.
My render settings are FFMPEG VIDEO with 30 fps.