I was a taxidermist for many years and we always did fish mounts in natural looking scenes and cliffs and here is one of the tricks we used and I bet It can be used here too.
We also went to the model train store and they have all kinds of really cool rubber molds for casting your own rocks and stuff so look in the train stores too for your aquarium
Step 1: get some scrap plexiglass for a base, also get a couple acrylic rods from local hardware store.
Step 2: Cut your base in the shape you want your cliff wall.
Step 3: drill several holes and cut your acrylic rod to the height you want your cliff and weld them into your base with acrylic or plexi glue.
Step 4: go dumpster diving at your local carpet store for some of their scrap carpet padding. you know the stuff made from chunks of foam rubber glued together to make the padding. (I always ask the store if they are open and they always say no need to hit the dumpster and take me in the back and hand me 20 times more than i ever need.)
Step 5: Cut the width you want your wall plus maybe 1/2 inch wider and do this until you have enough layers to make the height you want.
Step 6: TEAR DO NOT CUT the front of the padding giving you random shapes. your front will be rough like rocks and you want to tear like a wavy pattern to the front so your layers of rocks are all different.
Step 7: Randomly grab your layers and poke them onto the acrylic rods you might want to sharpen the tops of the rods a little to make it easier.
Step 8: once this is all finished you should have a nice stack of foam padding and it will look like a really nice rock faced wall.
Now for the Finishing of your Wall
Finishing 1: Use whatever aquarium
concrete you guys use and start out with a thin coat and completely coat the foam front and rear so that the foam is completely encapsulated. we used plaster of paris but in an aquarium
I doubt that will work.
I assume you can also use urethane's or plastics for making aquarium
stuff (maybe someone smart with aquarium
materials for these things can make suggestions.)
Finishing 2: after the initial coat has dried you can start layering a few thicker coats on until you get the desired thickness you want. Also you can use natural sponges while the coating is still wet and dapple it around giving the rock some more texture.
: for your final coat you might want to tint your concrete mixture so that it is a lot easier to paint to your desired color later on.
Finishing 4: add final painting and clear coat as desired. the plexi base is created with a couple feet will be able to be slid under your substrate and even put a couple regular rocks on top to hold your wall to the ground.