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.

Hey Cjloong...
My deepest condolences towards your dead shrimps.....

From what I can understand from you setup is...Your top tank with shrimps (higher grade?) over flow to the bottom tank (with shrimps too?) then only into the canister filter...So I assume that...The canister is the only filter you have for the setup with 2 tanks? Am I right?

For your situation, first clue that come out of my mind is, big lost of beneficial bacterial, causing bad water quality. But wouldn’t it be funny? I mean, if I’m not mistaken, those 2 tanks are planted, right? But are they heavily planted, with stem plants, or just basic moss kind of tank? That will be another question.

Just my thoughts thou...

Regards,
John

If your tank with plants and growing well. pump or filter out for 24 hours will be fine for them. Unless your tank overload of them. Mostly the bigger shrimp will die first. My filter was out for 3 days...just few bigger shrimp died only. The rest are ok.

Suspecting this...
2) Monday, my maid spray insecticide in her room. Outside her room directly is the aquarium.

Insecticide is very poisoning to shrimp. And you change water for 5% every 2 hours for 4 times. by using tap water or old water from other fish tank or the tangki water? As you knw, they are very sensitive to water change also.

If me, i will do 30% water change 2 times within 4 hours. So, part is yr maid mistake and part is yours.

Yes. To be more exact, it is 2 canister filter in series, one to house mechanical filter media and one to house bio rings. I usually clean the mechanical filter without affecting the bacteria.

Planted with mostly moss and fissiden. I guess that it is not really helpful for nutrient export compared with stem plants.

It is aged water, from a large water container. This container is filled by tanki water also. It is therefore, tanki water -> added anti-clorine and pH adjusted -> aged somemore... (I super kiasu on this )

It was until a day ago, the container have no more water due to excessive amount of water being used in the emergency. Now it is from tanki with anti-clorine while a new batch of aging water takes place.

Water change is almost dripping method controlled by a timer and pump. It is automatic.

Sigh, yesterday night, no electricity again for 4 - 5 hours. I hope the shrimps will be ok today. Have to wait and see tonight when I get home.

Seems like all bad things will happen, and will happen at once. sad... really sad.

My guess is it's the insecticide. I don't know how toxic insecticide spray is to shrimps, but I do know those mosquito mats contain active ingredients highly toxic to fish(allethrin). Shrimps are like underwater cockroaches anyway . That's just my uneducated guess... it just seems to abrupt a die off for anything natural to have happened. Assuming you removed the dead shrimp the days you discovered the dead shrimp... wouldn't die off en masse like that..

CRS too... ouch...

One of my guess is this...
Insecticide spray is oil base. Therefore it is floating.
My top tank have the overflow hole on water level, thefore the insecticide enters the overflow hole to the bottom tank.

-> Bottom tank have the most insecticide, therefore heavy casualty on bottom tank.

-> Heavy casualties causes NH4 spike causing some top tank CRS to die but not as bad. (2 on top vs 20 at the bottom)