Cementing ornamental rocks
If you should decide to attach the rocks, leaving aside the use of cement, silicon could be a good compromise. In this way you would obtain a junction that does not need long rinsing, and later can be easily separated with a knife, if you should ever need to do so. Fixing the rocks with cement, instead, gives much more stable results.
The cementing of ornamental rocks
For this operation we use a cement that is devoid of iron substances called Portland cement, which we will mix with double its volume of fine quartz. After the construction has hardened, we have to fill the tank and leave the water to extract the alkaline substances. We recommend a period of about two weeks, adding table salt to tap water. Salt water is, in fact, more chemically aggressive than fresh water. For the last rinsing we can use hydrochloric acid (muriatic), best if in a 32% concentration and a quantity of 15 ml for every 100 liters. With this method the remaining alkaline substances in the cement are neutralized. At the end we finish by rinsing everything with plenty of tap water. An analysis of the PH will indicate if the cement is still releasing alkaline substances.
It is also possible to create arrangements of different types. In particular, Japanese marine aquariofiles are masters of special arrangements, and their aquariums are notably different from most European and American ones. It is interesting to note how the majority of Japanese reef aquariums reproduce solitary coral formations, which we often see in nature. These rock coral constructions are positioned completely freely in the reef aquarium, and do not have any kind of contact with the glass, in order to allow the fish to swim around them. Sometimes for these creations scaffolding made of PVC tubes is used to avoid using cement. Individual rocks are then fixed to the structure with nylon thread or cable ties. In this way it is possible to create reefs with interesting shapes that are also stable. Some ingenious aquariofiles in the U.S. and in the Philippines have used this technique to channel water inside the pipes. From the start, the plan includes the possibility to later pump water in the system. Openings in several places allow circulation both under and behind the structure. No doubt, it is an interesting construction. However, it can be quite difficult to give a natural look to it. You should also add the risk that one of the main pipes might cave in under the weight of the rocks, inevitably making the entire structure collapse. Although this is not very probable, the risk of this eventuality should not be underestimated, considering the effect of rocks impacting the glass. In the end, whatever you decide to do to prepare your aquarium, whether it is the easiest of arrangements, or a more complicated structure fixed with cables or cement, with only living rocks or with a mix of the two, take your time. A well thought out plan is the foundation of a good aesthetic final result.