the first thing you need to do is to identify the size and weight of the tanks you want on the shelf. Water weighs 1kg per liter or 8 lbs per gallon. That is just the weight of the water. Tank substrate will add to that. So a small 5 gallon tank will weigh close to 50 lbs. Once you know the weight you need to find shelving that if rated for the weight. The the chelve needs to screwed in the wood studs in the wall. Sheet rock or plaster will not support the weight. Most people don't mount the tank to the wall. It its most often on a table with 4 legs. to transfer to the load to the floor.