[fishytime]

Your primary drain can be anything from a strainer fitting screwed directly into the bulk head, to a section of pipe, up to half way up the over flow. The bottom side of the primary drain, under the tank, should be plumbed down below your expected water level in the sump (with a gate valve). The emergency drain inside the over flow should be slightly higher than the operating water level of the tank. The bottom side of the emergency is left short of the expected sump water level (acts as the warning sound). The gate valve is used to restrict the flow in the primary drain, causing the water level inside the over flow to rise to just below the point where the water drains down the emergency. This means, in most cases, less than an inch drop into the over flow. I opted to plumb my primary drain half way up the over flow to make it easy to remove when cleaning. Don't glue or teflon tape anything inside the over flow as you want to be able to get that stuff out to clean, rescue fish, crabs, etc. Have fun!!!