Natural slate for aquariums, 1 to 2 kg

Always place the first stone on the bottom of the tank, before pouring in the sand. Sand that the fish dig through gradually hollows out under a slab placed on top of it, and the stack ends up toppling over. Once the base is resting on the glass, you can pile up two or three layers, crossing the slabs like the stones of a dry-stone wall, so that each piece rests on two supports rather than just one.

If you are using more than two thicknesses, or if burrowing fish live in the tank, fix the whole thing in place. Out of the water, a special aquarium silicone applied in dots between two panels is enough; in a tank that is already full, a glue that sets underwater saves you from draining it. Finally, leave a clear gap behind the structure, because sludge builds up in that corner and you will need to be able to get the siphon bell in there.

Several freshwater cichlids lay their eggs on a hard, smooth surface rather than in a cavity. This is true of the angelfish (Pterophyllum scalare) and the discus, which readily choose a vertical or steeply angled support. A slate tile leaned almost upright against the back glass gives them this surface, which is steadier than a plant leaf swaying in the current and easier to keep an eye on from the front of the tank.

Before spawning, the pair spend a long time cleaning the chosen surface with their mouths, then the female lays her eggs in rows which the male fertilises straight away. So place the slate in a calm area, out of the direct flow of the filter, and do not move it once this cleaning has begun. If you want to raise the fry separately, slate has a practical advantage: you can lift it out with its eggs and transfer it to a rearing tank without detaching anything.