I don't think it matters how large the drain pipes are. After a certain point, only so much is going "down the drain" to match what is coming in from the return pump. Either the pipes can handle it or they can't.
Based on how far my main drain was closed off with the ball valve, I could probably have gotten by with a 3/4" drain (even if I left my emergency a 1" pipe) and I'd still probably have to close that off a bit. A larger pipe (like 1.5") would just need to be closed further still.
One interesting thing about switching to a gate valve on my main drain is that I no longer have to also use the valve on my return pipe. The ball valve on my main drain was so hard to turn that I had to slow my pump down a bit as well with that line's ball valve. Now my pump (and return line) is wide open (increasing flow) and the only thing controlling all flow now is the gate valve on my main drain line.
I have it adjusted perfectly now. Its staying put at the water mark I taped on my tank (for checking my overflow level).
I purposely drilled for bulkheads on the back of the tank for using the Herbie. My overflow is on the left and return on the right. I put my main drain line about 4" from the bottom (with the 90 degree elbow and strainer turned down to where the bottom of the strainer is just off the bottom of the tank) and the emergency is about 7" above that (with the 90 degree elbow turned up). The top of the emergency elbow is about 1" below the weir of my overflow and the water level in my overflow is about at the bottom of the emergency elbow now. I have about 7" of water level to adjust in between, though I prefer it remain somewhat of a constant.
I should note that I siliconed my glass overflow into the corner at a slight angle (as I did with the baffles in my sump) so that no water falling over the edge splashes. It only gently cascades over it in total silence. The water doesn't really "fall". It just gently runs down the glass because of the slight angle. My system (both the display tank and the sump) is so quiet that if you closed your eyes you would think it was not functioning.
I split my return on the right side of the display tank from the 1" return line into two 3/4" pipe fittings using a reducer. It goes from the reducer down to a 3/4" "T" which is pointed down (causing water flow behind my rockwork down by the sandbed) and then I added a 3/4" 45 degree elbow onto the end of that (which is pointed up causing gentle turbulence on the display water surface, but also directs it towards the middle of the tank glass and then reflected back to the other side of the tank towards my weir).
In my sump, my drain chamber has the skimmer churning up the water, and both my refugium and return chambers have circulation pumps in them keeping the surface moving. Of course the refugium has the larger pump (for tumbling chaeto).
Lots of water movement (both in the water column and the surfaces), but no splashing or gurgling anywhere in the system. It is dead silent.
I have yet to test my ammonia. It has been a little over a week since I dosed it to 3 ppm and added the SeaChem Stability daily. Added the last of that today. I'll test my ammonia tomorrow and see where it's at. I only added the Stability to kick start the biofilter, not in order to rush things along, but because I am using all dry rock at this point. I would guess the ammonia level has dropped some, but I'm in no rush. Well,
I am but nature has its own schedule.
