overnight mass guppy mortality

[karen.nik]

i had 8 healthy adult guppies, all under 1/2” (all are still quite young survivors from 1 shipment over one month ago, and one shipment over three months ago), and one big mystery snail. this tank was stable until…

…i added three tiny horned nerite snails i got in the mail yesterday, 1/4 diameter each, in the morning when they arrived. they did not look lively, but the accompanying instructions from aquatic arts said to give them 24 hours to recover from the shock of travel, etc, and to put an algae wafer nearby before declaring them doa. 9 hours later at guppy dinner time, no signs of life from the new snails, and a couple guppies seemed uninterested in dinner, but otherwise ok. then this morning, EVERYONE WAS DEAD.

except the mystery snail. and it’s all a mystery to me. a very sad one. what in poseidon’s blue marble happened?

tank parameters:
ammonia: .75
nitrates: 0 ppm
nitrites: 0 ppm
temp: 75 degrees
ph: 6.0
tank size: 9.2 gallons
substrate: acrylic tank, no gravel
plants: guppy grass, moss balls, java fern, and dwarf hair grass planted in seachem aquatic soil contained in an unglazed, fired, traditional brown clay japanese miniature bonsai pot.
filtration: upettools sponge filters, two, each has one pair of sponge filters, and, in each tiny filter media compartment (one below each sponge holding approximately 1/3 cup of media) i have stuffed in a mixture of seasoned seachem matrix, and biohome bio gravel (this was a holding tank while i prepared a biggger tank with canister filter)
air: 2” hygge air stone, 1/2 full pump force
last water change: several days prior the below event, it was a 100% water change, and i rinsed the plants, and squeezed the sponge filters in fresh treated water
diet: consistently fed hikari guppy food

[Aiken Drum]

How did you transfer the snails from the bag into their new home? Did any of the water from the bag get into your tank?

If you are seeing mass die off of otherwise healthy fish then the likely cause is something got into water. Either something introduced with the snails, or its just coincidence it happened at the same time. Might some detergent have gotten in? Hand sanitiser? Could you have done water change and forgotten to use water conditioner?

I would say your tank isnt cycled. The level of ammonia you are reporting, while not going to cause immediate, overnight deaths, is a sign you arent cycled and could have caused longer term issues that went unnoticed until the fish died. You are seeing zero nitrate too, so its not like your tank crashed suddenly, it was never cycled. I dont know how high the ammonia had been historically before the deaths. Have you been testing prior to your issues? What sort of water parameters have you been recording?