So a little more thought about my constant water change system. Fresh aerated saltwater, properly mixed with distilled water (relatively cheap since it's produced and bottled here and guaranteed to test less than 0.9 TDS... and I can have it delivered!
) will drip in to the circulation pump chamber right at the pump inlet, where it will mix thoroughly with the tank water. As the water level rises in the pump chamber (and falls, since it's a surge tank), excess water will splash down into a drain chamber and run away through a hose to a disposal. The amount of water changed over time will equal the water dripped in, as long as the drain hose doesn't become plugged (and assuming 0 evaporation, or a perfect amount of makeup water added constantly).
This system isn't foolproof, I can already sense that I will have some issues with 'salinity creep', though I can't really say how bad it will be. I may be able to get away with mixing my change water with a slightly low SG (since the bucket will be sealed evaporation from this container will be negligible). I haven't decided if I'll go open top yet, since I haven't planned to have fish I just may do so.
And I have a question to post to you DIY types... how thick do I have to go if I do acrylic? I'd like to use 1/4", but don't want to brace and I worry that with the 24" span of the main panels, I may run into a problem with bulging near the middle. I'll go thicker on the long sides if I have to, I'm just not sure if I'll need to.