Drilling a tank

Since i have such a high nitrate problem im gonna add my 10gal sump i made a while back. I was gonna replace my current 10gal and get a new one. Which i was gonna drill the back of the tank so i could incorporate my sump. what size would the PVC pipe need to be? and the hole? I might just get a bigger tank for my Display tank. But id hate to buy a new stand.
 
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