Well, all the BTOEs dropped their eggs, as usual. But, I just figured out something today that makes the picture more clear.
Our water is softened.
Yup, didn't know that. After being in this apartment since late September, I have wandered in and out of the community laundry room, which has the massive water heater, on a weekly basis and never bothered to look at it. Today, in my boredom as I waited for the washer to finish, decided to look at said heater. Turns out it's also an industrial water softener.
That answers A LOT of questions. About everything. Why my corys couldn't take it here, why my plants look like crap, and why my BTOEs and CRS won't keep their berries.
So, I'm actually starting to use RO water again. I've talked about it for months, but now I'm actually doing it. I got some Fluval Mineral supplement crap that fails with instructions. So I got out my trusty not-calibrated gH test, a gallon of RO, and a bucket and went to town. Turns out 5ml of that stuff increases the gH of one gallon of water by 13 degrees. I added 7.5ml of that stuff (that only says it has calcium in it), and 1/4tsp of Epsom salt, which ended with a gH of about 30 degrees. So now, my weekly water changes will be with one gallon of straight RO plus two cups of this "concentrate" mix. This gives me an approximate 4-1 Ca to Mg ratio, with my parameters leveling out to about 3.75 dgH and still 0 kH. It'll take a while for this water to really have an effect, but it gives the shrimp ample time to get used to the new parameters.