I wouldnt return it with out trying some more practice, unless you have the money for a nice SLR ( I dont LOL). I tried a few different point-and-shoot cameras in the 200-400 range and this is the one I had the best luck with.
As far as settings go, I think there is only one way to take macro pictures (mode with the little flower icon) with the Fuji A350. I dont think you can focus maunally in macro.
I just hold the camera right up to the glass or a few mm away (as perpendicular with the glass as you can get it or it will be distorted)
Set the flash to auto or off (off works best for me, I let the tank lights do the work)
The macro is only good from a few inches to 12 inches away IME, maybe slightly more with good patience.
Try pressing the button down half-way a few times once you have your subject, this will help to get the lens to focus and give you a better idea if you have a good angle and distance. Once it is in focus with the button half way down, take your picture.
A tripod will make getting things in focus much much easier as long as you can still get the camera up to the glass. When not using the tripod, I rest my elbows or the camera on the tank trim)
I'll try to take some new shots later today and see if I can get any top down shots since the glass of the tank takes out som much of the bright colors.
Best of luck!
Wes