Hi all
Recently, I have heavy casualties on my CRS population. Previously, I have 20 shrimps which grew to be a population of 80 - 100. I was looking into doubling it by June/July but accident happened. 3 things I could think of that may cause shrimp death.

1) Last week, electricity kong out for 7 hours. Suspect that cannister filter bacteria all died.
2) Monday, my maid spray insecticide in her room. Outside her room directly is the aquarium.
3) My chiller konk out last week as well. Temperature have been 27 - 28 after that.

Sequence of death.
1) Last week until yesterday, 1 - 2 death per day.
2) Stopped last Friday, no more death.
3) Monday, saw a lot of dying shrimp after dinner. Change 1/2 of the water immediately and 5% every 2 hours thereafter 4 times.
4) Yesterday, 20 dead shrimp.
5) Today, wah... 10 more dead shrimp.

My tank setup is like this.
1) Top tank overflow to bottom tank from water surface (not bottom of the tank).
2) Bottom tank pump to canister filter back to the top tank.

Last week, casualty is in the top tank, my higher grade CRS.

Yesterday, bottom tank suffers heavy casualty.

Just would like to brainstorm with you guys on what is the most likely killer and what could have been done to prevent the deaths. Like to learn as much as possible so that in future, will know what best to do.