So I eventually found out it is called a “spring box”. And no, I saw no discernible change in level from June to end of Nov. I checked after some monsoon-like weather. I think this is supposed to be good in that the source of water is probably coming from way up in the mountain filtered through the earth over time vs surface water quickly finding its way into the box. At any rate I plan to put in a UV and other filters. At some point I will probably redo the whole thing so I have some more waterflow and don’t have to use the suction.