Shrimps mass death

[Forsalmon]

Around 50 cherries died last night. I have kept them for more than a year, and I have not been able to pinpoint any reason.
Tank is more than 1 year old, due to algae issue, I took out all the algae infested plants last week. Added in new plants yesterday, and shrimps died overnight.
Prior to the mass death, I noticed that the shrimps have been gathering at the top corners of the tank. At first I thought that they were trying to eat the algae there, but it seems to point at low oxygen levels since there were no more plants in the tank.

Water parameters: ph ard 6.8 to 7.0, 0 ammo, NO2 & NO3.
Tank size: 1.5 ft
filter: eheim 250
tank mates: 2 oto, 2 cory, 8 guppies, 20-30 shrimps (after that mass death)

Ignoring overcrowding, what other causes are there? Overcrowding won't cause an overnight mass death. The new plants are all bought from seaview, slightly washed with tank water.

[Urban Aquaria]

If the deaths occurred shortly after you added the new plants... then most likely those new plants still had residual pesticides or chemicals on them that caused the mass die out. The pesticides and chemicals will often have fast kill effect on the shrimps (they will all swim to the surface to try and escape the toxic chemicals), but the same toxins will have no effect on the fishes.

This is actually a very common issue and happens to alot of shrimp keepers. For shrimp tanks, you definitely need to wash all new LFS plants thoroughly and quarantine them in a separate tank running with activated carbon for a while (with lots of water changes to dilute the pesticides/chemicals) before usage.

[Forsalmon]

Is it too late to take them out now?


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[Urban Aquaria]

Is it too late to take them out now?

Well, since you still have some shrimps in the tank, it would be best to remove the plants and do the additional washing and quarantine procedure to clear as much of the residual pesticides and chemicals from them.

Do a series of water changes in the main tank to dilute the toxins still present in the water and perhaps add in activated carbon to help adsorb the toxins too. If the remaining shrimps start to show symptoms of poisoning or stress (they move around unsteadily or flip over or crowd at the water surface), quickly shift the affected ones to another tank so that they can recover.