Lately needed to make a rather high slope 22 cm all the way from the bottom going emersed.. And i found an old bag of filter lava dark color (not red) in the shed i used in a pond filter. Irregular shaped chunks of lava ranging from 15 to 40mm in size. You can stack it into a pyramid for example and it hooks firmly togehter, no glue needed. It stands on its own.. I used this to bank up what i needed and capped it with a 3 cm layer of fine gravel. Where the bigger plants should be planted that need to be planted deeper i left some pot hole in the lava base to be able to plant a tad deeper.
The plant roots will grow into the porous lava and hold it even firmer together. It will never ever slide. Maybe the top layer if you flood to soon before all carpeting is rooted properly.
imho using lava has some more intersting properties, a pile of stacked lava rocks provides a labirynth of nooks and cranies. I made cave structures with some decorative rocks and didn't cap the inside of the cave. To provide an antrance to the lava labyrinth for shrimp fry and other micro organsime.
I got this idea with monitoring micro orgasme behaivor in my garden tubs.. At times they looked empty, but looking realy close they where still full of live but all was where the food is close to the bottom grazing the detritus. The watercolumn didn't provide food, the detritus on the bottom does.
Now i threw a lot of that organsime into that scape (still fishless and starting up). And indeed 3 days later i saw non of it back as iff all were gone. Dropped in some more and again few days all gone again. But they aren't, they are in the lava labirynth where the food is. Occasinaly looking close i see some in the water column, but the number in there is far greater than what i see. The good thing is, once/if fish are added, they can not get to the micro organisme, only what dares to come out of the labirynth serves as food. The rest can go on do their thing and propagate undisrupted.
🙂
This eliminates the theory not creating open pockets in the substrate to prevent detritus accumulating and rot. Maybe true if you hermetically seal off the pockets with a capping. But if you make sure it aint hermetically sealed off so micro organisme can get in, it only adds to the biological diversity of your aquarium rendering the detritus in your substarte to something usefull.
That's the only way to for example keep Daphnia, Rotifers etc. population alive and breeding in a tank together with fish.
Sure, you can use other materials than lava to adchieve the same structure.. Lava has my personal prefernce because its easier on the plants rooting.
🙂