Join Date: Nov 2004
Location: Puget Sound Area, Washington
I recognize that planted tanks tend to require relatively little water flow (compared to marine), so you're probably thinking of erring on the small side. However, if you find that your bulkhead provides too much flow, it is easy to restrict it (1" to 3/4" PVC adapters are almost free). The opposite is not the case; so, I would err on the side of excess bulkhead capacity.
On a 180G tank you probably want at least one, probably two, 1" outflows. Another 2 for return (if you're drilling bulkheads for the return).