Freshwater Shrimp Dying

So for the past few weeks I've had my precious freshwater dwarf shrimp die off, one by one. Only one death a day every other day, but still, it's disheartening. The other shrimp are not active, not eating...It's been heartbreaking.

Not only has this been an extremely expensive loss (they're $5/ea!), but also a puzzling one too. I have no clue why they are dying! My best guess so far is: a bacterial disease.

It's not my water quality- which is pretty much perfect.

6 gallon cube aquarium
73-74F
0 ammonia 0 nitrite <5 nitrates
KH (Carbonate hardness) 2
GH (General hardness) 8
Total Dissolved Solids (TDS) ~180ppm
15-20% water changes 2x a week
Densely planted like a jungle and plants are thriving

Most of the shrimp that have died already were actually my red cherries and painted fire red shrimp. I must've lost all my crystal black shrimp too along the way because last week I checked everywhere and couldn't find a single one left (I had 10 a couple months ago). So on top of the lifeless carcasses I've been scooping out with semi regularity there have been several other deaths wherein the bodies were cannibalized in the middle of the night. :'(

I've since added Seachem Prime to the water (in case it's a problem to do with ammonia/nitrite/nitrates, which I doubt) and a big catappa almond leaf (supposed to help naturally treat bacterial disease). A week of doing this and I still lost 1 shrimp that I know of. Some days the remaining survivors are active and eating, but most days they are still listless.

My other small 4 gallon tank (where I relocated some shrimps, hoping they would heal in a different tank) isn't doing so well either. I lost 1 shrimp in that tank last week. Current survivors are not looking good. Pale colors and barely moving let alone eating. My dwarf orange crayfish in that tank also mysteriously lost a main claw so now she's hobbling along with only one pincer. Breaking my heart...but I read that they can regrow limbs with time. (Please be true!)

Meanwhile in my 36 gallon, green spot algae is running rampant and I lost 3 fish this week - my longtime starving killifish, a red calico platy that started getting sick out of nowhere, and I lost 1 dwarf rainbowfish 2 days after adding the school of 4. I must've treated this tank 4 times already and fish are still dying from what I suspect is bacterial disease or maybe even parasites?

I hate how it's almost impossible to diagnose the true cause of a fish's disease. It's frustrating and extremely annoying. I'm praying that more frequent water changes will make the remaining fish healthier.

So it's just been one piece of bad news after another I'm afraid. I'm starting to lose confidence in my fish care abilities. I know, I know, having deaths is an inevitable part of the hobby but I feel like I've been on a death streak with no end in sight.

To further exacerbate matters, I am going on vacation soon and the only person I can rely on to take care of my fish and plants is my teenage brother who knows nothing about these things. I am terrified he will end up overfeeding my fish or polluting the water somehow and I'll come back with tanks full of dead fish and rotting plants. (*freaking out*)

Well, hopefully I'll have better news next time.