The deeper the tank the greater the pressure. If the tank was only 6 inches deep there would be very little pressure on the eurobrace or whatever was bonding it to the tank, 18 inches deep there would be more pressure on it, 36 inches even more yet, and so on. The longer a tank is the more it's going to want to bow, there's a lot of things to factor in. If the bond between pondshield/epoxy and silicone isn't as strong as that of glass to silicone there could be problems if using silicone as your bonding agent.
I tried starting a similar thread several days ago and it went nowhere. You've got to be very specific,I tried but people still didn't seem to get it. Are you trying to silicon in a glass or an epoxy covered plywood brace, if plywood, will there be screws involved, that type of stuff as specifics people will need to answer.
I was trying to find out if a silicone to epoxy bond was as strong as a silicone to glass bond, didn't get much help. Using silicone to glue in a front window is more or less using a sticky gasket, the glass is held to the front by water pressure from within the tank as much or more than by the silicone itself, using silicone to bond in a brace after the fact is potentially VERY different. If it were equally effective, I'd just caulk in a glass eurobrace after the window is in. If it were equally effective you could in theory epoxy some plywood, edges included, and just build a tank with silicone caulk joints without any screws, just like a glass tank. The standard plywood build typically seems to be with plywood eurobraces AND cross braces. It's tough to tell if it's because other ways won't work of if that's what's been tried and worked. Somewhere out there there's probably someone who's tried doing structural work with a silicon to epoxy/pond shield bond, but it's not the typical build.