Turns out my fifth shrimp didn't die after all (I looked up the lifespan- they can live five years or more!) so what I found must have been a shed exoskeleton, one of the largest and most complete I'd ever seen thus my mistake. I started dosing the tanks for internal parasites today. Striped kuhli Sassy is looking thin- the head definitely bigger than the narrow body, where all the others have nice girth. Conflicting info online about whether General Cure would harm shrimp so better safe than sorry I pulled them out. Set up one of my little plastic tubs with a few gallons of tank water, small sponge filter, bit of fake decor for them to hide/climb on, and a handful of gravel for the good bacteria.
It was not easy to catch them. I'd seen in someone's video how they catch shrimp by simply placing the end of a wooden skewer in front of the shrimp which would climb onto it and then he just lifts them out. Mine would not do that. Every time I approached them with the tip of a skewer they move away. Then I tried sinking small clear plastic cup to lie on the bottom, and gently chase the shrimp into it. They are very quick and skittish though. Easiest way turned out to be just guiding them into open areas in the front of tank, and then catching from behind with the cup. And I found all five.
Fishes proved they have very good memories. To my kuhli loaches it must have looked like I was placing a food trap- they swarmed all over the plastic cup when it was on the bottom, going in and out inspecting all the corners, it was actually pretty funny.
I decided to dose the window tank as well. Last time did a cleaning siphoned out a lot of pale/white poo. It could just be from what they eat- I've noticed the swordtails pick a lot at the white scum that grows on the rubber base of the heater. But also could be internal parasites, so not taking the chance.