I have the 5700 and a D70s,
First thing you will notice on 5700 is that it is too slow to focus and will have a considerable shutter lag.
If you use automatic focusing then it might focus a bit faster however would still be slow and sometimes would focus on other thing than what you want.
considering the above you are better of trying to have the camera on tripod, compose your image according to what you want it to have in the frame, prefocus and wait for your fish to go in the frame. if you wait for the fish to go in, by the time the camera focuses then followed by it's shutter lag the fish will have most probably moved out.
Don't get me wrong it is a very nice camera and I stil have and soetimes use mine.
It does not have an autofocus assist lamp which makes focusing harder in low light.
Now for the good points it is a great camera especially for macros. Choose macro mode (flower sign) and zoom in to midrange yo uwill see the flower sign turn yellow in an area just a bit after the W (wide)and till just a bit before the T(Tele), you want to have your zoom in the area where the flower is yellow as this is the effective macro mode. In this mode the camera focuses at 3 cm which is great for magnification levels.
Other than that the camera behaves as all others, yo uwill have to learn how to use your S mode or A mode for different shooting This camera's limitations (focus and Shutter lag) will lead you into shooting more macros andclosups of still life i your tank, use a tripod and follow general guidlines for composition and DOF on such shots and you will enjoy it.