#surgetank

Put a 3″ (or larger) schedule 80 bulkhead on the floor of the fuge with a threaded fit inside the fuge. 

Outside of that bulkhead, Tee into the fuge overflow’s drain pipe and send that into the display, well below the water level, preferably very-near the bottom of the display. This brings the normal fuge output (pods/etc) in near the rockwork (not over the display’s weir) and when creates an upwelling current that I really want, especially during a surge.

Inside, fit a ‘standpipe’ in the 3″ bulkhead to the height desired as the low point during the surge, I’ll set this so there’s 50 gallons above this standpipe that is below the fuge’s weir. I want the standpipe threaded into the bulkhead so I can change the max-surge when I want to.

Fit a 3″ toilet flapper to the top of the standpipe so the rubber flapper seals against the top of the standpipe (note: account for the added standpipe height when planning the standpipe length).

Connect the flapper chain to a toilet float ball. 

Note: If you stopped here, you’d have a pretty normal borneman except it’d be in a fuge. One added note. I’m expecting the fuge’s water level to be about a foot above the display water level, but its floor to be below the display’s water level. I’m actually planning that the 50 gallon surge will be the height difference in the two water levels such that the two tanks will have exactly the same water level when the fuge has dropped about 50 gallons. All this means the standpipe can be a tad short and it will never actually run dry; I’ll never have any noise or bubbles. Downside is the float may need to be made heavier so it actually seals in this case.

Scrap the toilet float ball, get a plastic rod long enough to extend well above the water from the toilet flapper

Connect the plastic rod to a linear actuator.  Exact actuator spec shouldn’t be too critical, but you may need to compensate for the total lift height with a fulcrum.

When the actuator lifts the seal, the surge starts. flip the voltage and the actuator seals off the surge.

This means the the water level in the fuge’s overflow box will never drop all the way (unless the display tank’s water level drops well below the weir), which in turn means the pipe will be always full of water, never losing it’s ‘siphon’ because it really isn’t much of a siphon since no part of it is ever above the water level. I was planning to have a pipe going up into the overflow box so the level didn’t drop below the u-tube’s outlet, but that’s not necessary since the water level can’t drop anyways. I just make sure the u-tube outlet (in the overflow box of the fuge) is below the display tank’s water level. the ‘surge’ pipe can just end in bulkheads on either end and will never have air in it, therefore no bubbles and no sound. I may get a little air/bubbles from the u-tube as its siphon starts/ends, we’ll have to see about that, but it must be better than what I was expecting.

I’ll still put it in a bubble tower to mitigate bubbles we may get, and to prevent sand and critters from going through the pipe.