4.99?! ITS FLIPPIN' 10 POUNDS for a kilo here in england!! D:<
As for the way of rocks - I personally like the look of having a build up of rock on either side, creating a 'V' shaped valley in the middle, that way you will have different fish in various different places, some on one side of rock, some on the other side, and then the other fish who prefer the empty middle section

Hope that helped!
You could also try making an 'island' in the middle of rock, where the fish will swim around it, and others will hover over it.
Or the classic wall, where fish will be everywhere, which is what most people (including myself) do/have done.