Casualties :(

[LittlePenny]

I set up a second tank in August and let it cycle for several weeks. When there was a visible biofilm I ordered 10 shrimps and added them to the tank (exactly a month ago). The shrimps were doing great and I found multiple molts shortly after, but today I noticed 2 shrimps on their backs, dying. The snails seem fine, I was able to see 3 more active shrimps that seem fine (not sure if the remaining 5 are alive or not, cause I created a hiding spot). I have no idea what could have caused it. The tank has a lid that has been closed (I open it to "air out" every few days).
Not sure what to do should I transfer the survivors? thank you in advance for your help.

Note: I didn't transfer anything from tank #1. The problematic tank is the new one, with all-new shrimps and snails.

[SuperProofer]

Are you sure they were dying? Maybe they both just very vigorously molted and were exhausted? Otherwise, is it possible some kind of airborne contaminant found its way into the water?

[LittlePenny]

Thank you for the response, SuperProofer. They have been on their backs for 2 hours now (since I've noticed). I am keeping an eye to see if they bounce back (or get eaten ). The only thing I can think of is that the cleaning lady was here yesterday, but I had the lid on, and a post-it note to please not move the jar. I will post an update later on.

[LittlePenny]

UPDATE:
One confirmed death
I was able to catch 5 and put them in a temporary bowl, and did a water change as an emergency measure for the remaining 4 shrimps that were too quick for me to catch. The plan is to get the remaining 4 out of the tank into the quarantine bowl, and boil everything in the tank and start over.

[Kurai61]

That really sucks Penny! Here's hoping that the rest of them will be fine...

[LittlePenny]

Thank you Kurai61!

[Dch48]

Personally I would stop completely covering the tank and allow for air flow at all times. A lid with holes or slots in the sides works well. Nothing can fall in.

[JDillon]

Can you post a picture of the tank that’s having issues? Odds are something in it is causing the deaths. I had a couple die on me when I put a sea fan in their tank. I think they were eating something on it that killed them. After removing it, no more deaths. So as pretty as some items are they probably don’t belong in a brackish tank.

[LittlePenny]

Thank you for your responses!

Dch48 - that's a great idea, I will drill a few holes in the top.
JDillon - The tank only has a few dragon rocks, plastic plants, and some black sand (not live) -all were boiled and rinsed prior to the tank set up. I let the tank cycle for nearly 4 weeks before adding the shrimps, and they seemed great for a full month before started dying

[Vorteil]

So only 1 confirmed death? If so maybe you're over thinking it? You have no idea on the history of your shrimp, maybe they died of old age? or maybe there is something in your jar that is producing toxins in you water. Plastics to me are kinda sketchy. Maybe that's the culprit?

[LittlePenny]

Vorteil - I didn't think about the plastic plants. Good speculation, I should remove it. About half the shrimps were dying (laying on their backs slowly moving their little legs) but I was able to get them out in time to a bowl and they bounced back, except for the one who didn't make it.
I am working on starting the tank over, hopefully, it will go better this time around.

Thank you all for your suggestions! I really appreciate it

[LittlePenny]

UPDATE:
I left the shrimps in the bowl I put them in to rescue from the harmful tank, while I was waiting for the re-set tank to become less cloudy. Yesterday, I went to transfer them into the cleaned tank and found another dead shrimp in the temporary bowl. I have no idea what caused it, as the water was freshly mixed (not taken from the tank since I didn't know what caused the mess) and there was nothing else in that bowl. I transferred the shrimps and snails back, hoping they make it