To prevent clogs I would say go up 1 inch. Increasing hole size in cap will NOT raise level in tank. The height of water in the tank is controlled by your pump. It might be oversize and need to be valved back. If done over again I would go at least 1 inch bulkhead but you have what you have. You have plenty of holes drilled in your pvc inside the tank, the only restriction is the 3/4 inch pvc.
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