welcome to this small guide to how to understand and use the Custom Rocks pack mod.
after going through this tutorial you will be able to create creations such as this:
let's go :)

In total this mod features 6 heights:

each height has 22 variations spread over 6 different forms:
sand center (1 variation)
fully covered (3 variations)
3 sides (3 variations)
2 sides (3 variations)
corner (6 variations)
straight (6 variations)

in total the amount of custom rocks is 132.
the pieces with rocks are obviously for the sides but the sand center pieces are used to fill in area's where rock sides aren't needed.

sadly due to the coding you can't just place a rock formation in the tank and a piece of scenery on it.
as you can see, the placement marker turns red, so if the rock is already in the tank, scenery can't be placed on it. However there is workaround.
as you can see, the rock formations can also be placed on the floor where visitors and staff normally walk. through that it's possible to place scenery on a rock, as you can see, the placement marker is now green. So simply place the scenery on the rock formation and then select the rock formation and move it inside your tank and tada, your custom rock now has scenery on it while it's in the tank.

Due to make sure all rock sides connect with each other(though in very rare cases minor gaps are there, if that annoys you use another rock formation that connects better). the rocks will clip through the sides of the tank.
As you can see in the screenshot it's clipping. Some may not find it annoying, others however might find it annoying. this can be solved in 2 ways:
1: use tank equipment/platforms or stairs to hide them from sight, this works well with the rock formations that are quite low such as 0:25, 0:50 and 0:75, for those that are higher i suggest you use method 2.
2: use the Ultimate building kit by Toxo and Bosjes https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=1813848426&searchtext=building+kit
as you can see the rocks that were clipping are now hidden from sight.

i hope this tutorial was of good use to you, i can't wait to see all your creations on the megaquarium community discord.

With the mods latest version: 2.0, all custom rocks are now paintable, which means all surfaces which were first just sand can now be painted to any surface you want.
if you want sand for a saltwater tank or perhaps dirt for a tropical terrarium, now its possible, go for it.

as you can see, all of the custom rocks will now start as this, same paint as the floor.

simply select a paint in the paint tab.

as you can see here, it will offer new forms of customization to all of your enclosures.

simply go as wild as you want :)