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Generally speaking, those types of PVC pipe overflows don't work very well and are prone to cause flooding. The main problem is air getting trapped and the siphon not restarting.
You MIGHT eventually get it to work for a while, but they usually end up failing at some point.
You best bet is to drill the tank. If that is not an option, then a U-tube type overflow by Lifereef. eshoppes, or the like would be a better choice than what you have.
Sorry to be the bearer of bad news, but been there, done that, got the wet floors to prove it.