Yet Another RCS Croaking Thread, and CO2 help...

Alright, so I'm new to both planted tanks, CO2 and shrimp keeping, so I'm stumped and there's likely an easy fix. Help! So, as much stats as I can muster:



Fluval Edge tank, 6gal, default filter
CO2 (I'll get to this in a second)
Lighting is a complete copy of this mod, with an Ikea drop light on either end to light the darkest edges
Temp 77-78f
Nitrite, Chlorine = 0
Nitrate = 10 (could probably use more for the plants? Do I just be less diligent about poop cleanup every week?)
GH = 90
KH = 120
PH = 7.8

Critters:
galaxy raspbora
1 amano, 3 orange, 1 CRS, 1 vampire shrimp, dwindling RCS
2 nerite snails
4 assassin snails
(the one CRS is because my LFS had CRS and RCS in one tank so when I got them one gained white stripes, haha)

They all get a tablet of bee shrimp food whenever the last tab is done, and a daily shake of bloodworms for the fish.

Dosing:
primed tap for water changes, I know this is probably a problem causer
1 cap excel daily
recc'd dose of flourish trace and nitrogen with water change
20 drops phyton git with water change
*NEW* 2 drops ECA daily

Tank has been running since may. While the RCS have always been deep red and constantly picking around for food as they should, they've never made babies for me (I was thinking of getting the shirakura liquid mineral if it's a soft water issue). A few weeks ago one had berries. So excited!
Meanwhile, I've been trying to dose CO2, and I've been using a mini Pierce System up until this point that I've slowly been replacing with ADA as I can afford it. Problem is, I can see the bubbles getting spun into the tank and dissolving, but my ADA dropchecker still reads deep blue no matter what (I've had it near the bubbles, across the tank, this never changes anything). I've tried 1b/3sec, 3b/1sec and variations between with no nightly turn off, 1/3s doesn't spit anything into the water and the 3/1s blows my mini canister in two weeks (and, alas for tiny apartments, I have no space whatsoever for any canister larger than the tiny one sitting on my kitchen counter).
So the last time my CO2 ran out a couple weeks ago, I decided to try without and just dose ECA since my drop checker said I wasn't doing anything. I would guess I was doing more than it thought because my HC immediately began to suffer (hard to see, but there's browning in the underleaf areas):


After these two changes, I got an RCS that died. Alright, the others are bright and still happy. Then several days later, another. This morning 4-5days later, yet another belly-up that the assassins are chowing down on. Here I thought my CRS would be the little tank canary, but he's got blinding whites and is as happy as could be while the RCS are dying.

(and of course with the CO2 momentarily gone I get hair algae)

That's a lot of CO2 talk but my point is, my shrimps are dying but only my red ones and I'm baffled. I've heard assassins are fine but I've also heard they'll pick up a meal if they're hungry, do they need more food than the bee shrimp tablets (I think their snail supply ran out)? I want them all happy and making more babies than I know what to do with. Help!