Well, it's in there now.

It does slow down the filter a bit, which isn't a surprise since it is putting a greater load on it, but It still pulls water through it at a fairly good rate.

I tested the water today and even though it was neutral yesterday, it's back to super-high pH today, which is what I feared would happen. The reason I haven't tested the water until now is because 10 years ago I had an aquarium and tried my best to keep the water in good condition but found that nothing started to live until I gave up and just let it be. In particular, I'd check the pH every day and adjust it and the next day it was right back where it was. However, trying to change it every day kills the bacteria that the tank needs to process fish waste, so they actually did better when I gave up and just let it be because then the bacteria could thrive.

The flower vase on the right I put neutral water in two days ago, and also put peat moss in the paper-towel filter you can kind of see above it. I tested it too and the water hardness doesn't seem to have changed much if it changed at all, but the pH hasn't gone up nearly as much. ...and the internet tells me that peat moss lowers pH. So I decided to go ahead and put it in the main tank despite not having seen it lower hardness yet.

The peat moss does turn the water an interesting tint. While it gives it a bit of an unclean look, I do kind of like the color, and I think I'll especially prefer it in the main tank where the "cool white" LEDs are honestly kind of blue. Perhaps the two will cancel each other out nicely.

I did try a paper towel on the front of the peat moss box, but it would quickly clog and block water flow, whereas without a paper towel I still had water flow a few hours later, so until I come up with another idea I'm just going to see how well this works.