Shrimp dying, everything else (including snails) fine

So this is a problem I've had for a couple of months and I'm about to the point of tearing my hair out.

I have two tanks (well, 3, including the girlfriend's betta tank). The "big" one is a 20g with a canister and UV, and the other is an Ebi. Both are heavily planted.

In the bigger tank I have cardinal tetras, oto cats, dwarf cories and a baby blue ram, all of which are happy and thriving. I cannot keep shrimp alive in this tank. I had a colony of cherries in there which, after a month or so, all up and died over the course of two days. Since then I've introduced ghost shrimp and amanos, which have also died within two or three days of being introduced. I have MTS and nerites in the tank, all of which are doing fine. There's definitely detrius/algae/food for them. Water tests fine (6.5pH, 0 ammonia/nitrites, ~10 nitrates, no color at all on a copper test, including with double the drops). There's an air stone in the water and good water flow. Water changes are about 30% weekly, conditioned with Prime.

The Ebi was started with about half of its water from the other tank and the other half conditioned tap water. The cherries and malawa shrimp in there are doing swimmingly.

So what's killing the shrimp in the first tank? It's not the water, it's not anything toxic for invertebrates in general...the bodies show no sign of being harassed or attacked by the fish in the tank, nor have I seen any such behavior. And the last ghost shrimp I had in there were pretty big, as big as a large amano. I've searched everywhere for possibilities and can't figure it out. Any suggestions would be greatly appreciated.

Ah, I hadn't found Prime as a possible culprit. I thought it might have been the CO2 Booster (API's version of Flourish Excel), because I'd found threads where that may have killed shrimp...but I've since stopped using it (went to DIY CO2, but at a very slow rate, and with the air stone going off every 90 minutes for 15 minutes to make extra sure there's no oxygen problems) and added an air stone just to make sure oxygenation isn't a problem. I have plenty of Stress Coat, I could try switching to that (I'd been using Prime because some plant people don't like the conditioners with extra "stuff" in it).

The plants are pretty similar in both. The big one has dwarf hairgrass, baby tears (the stem plant, not dwarf baby tears), dwarf red lilies, a banana plant, a couple of different crypts, brazilian pennywort, java ferns, a couple of different kinds of moss. The little one is more low-tech and is mostly mosses and baby tears.

I also thought the fertilization may have been the problem...I was doing EI with bigger water changes. So I stopped fertilizing temporarily. Since then I've taken some of the water out to start the Ebi and added ghost shrimp to the big tank...the shrimp in the Ebi are fantastic, the ghost shrimp died in 3 days.

Maybe it's just the cardinals and/or ram harassing them to death. That seems to be the only variable (apart from the not very likely Prime + my tap water - plant that one aquarium has that the other doesn't that absorbs the whatever the combination produces). I just figure I would have seen it or at least see evidence of it.

The heavy metals theoretically shouldn't be an issue, because I handle the water in both tanks the same way (and the tank the shrimp survive in was started with a bunch of water and filter media from the other).

Nitrates are low in that tank, especially since I've stopped adding more as fertilizer. I've dosed a little bit of reef iodide in both tanks (1/10 the reef dosage per water change). I usually drip acclimate. The ghosts I just netted and threw in. The pH in both tanks holds around 6.5 or so, there haven't been any wild swings. It's a tad low but nothing they can't acclimate to.

And harassment seems really odd, too...that tank has tons and tons of hiding places...rocks, plants, etc. Yesterday I found 5 dead ghost shrimp all right in the middle of the tank. Haven't found the 6th one I added, I'm kind of just assuming it's dead somewhere I can't see.

Just a guinea pig, no fleas there.

I forgot to mention that I put a packet of Purigen in a few days before I put the last shrimp in. I just put a few more ghost shrimp in yesterday after doing a big water change with Stress Coat. We'll see.

Well, a week in and at least 2 of the ghost shrimp are still alive (they're **** hard to find), and I haven't seen any dead ones. So that's good.

Yesterday I put the mother of all Amano shrimp in, keeping fingers crossed.