My tips: forget about flash, depending how fast fishy objects are adjust shutter speed to freeze their movement, avoid dark areas of the aquarium. I'd go for S mode (around 1/200 sec.) and Auto ISO. If your lens is not very sharp wide open go for M, f/2.5, 1/200 sec, ISO setting depending on light conditions and 3D tracking AF if there will be no reflections on the aquarium glass. CPL filter would be ideal but will step down your exposure.
35 f/1.8 is not a portrait lens but you can use it as it's fast. 105 and 85 mm will be to narrow for bigger objects.