You can seal it with Drylok but it will still float. If you put concrete on it the weight will keep it from floating but it still won't be stable. I don't think any amount of velcro will hold that up, so silicone or some sort of bracing system would be necessary. I'm not using concrete on mine, just sprayfoam, so I think velcro will hold it in place fine. If I were doing a concrete one, I think I'd try to engineer something to hold the bottom in place, keep it from sliding out. Maybe brace it with some heavy plastic dowel rods running under the sand that press against the front of the tank. At the point where the dowel ends contact the glass use some sort of rubber padding to keep the rod from slipping and to help distribute the force better. At the top maybe several big plastic C-clamps to keep it from tipping over. I have no idea if any of this will work, I just made it up, but it sounds good.
I think I came up with a solution for the silicone and velcro. I'm going to sew the velcro to some of that plastic mesh canvas they sell in the craft store. The plastic should hold the silicone better to the glass.