moved molly fry & they died

[millym]

hi & help
we had 10 molly fry of varying sizes & they were in a breeder box in our main tank all looking quite happy. on the weekend we decided to get them their own little tank as it was getting a little crowded in the box. we set up a 18X10X10 tank with 1/2 gravel from the main tank, 1/2 new gravel- thouroughly washed,main tank water & rocks from main tank. we also put in a fluval 1 filter. i added some filter media from our main tank in this little fluval.
our babies looked quite happy until yesterday afternoon when some of them started to sit on the bottom of their new home with their tails sort of clamped together. a couple died v quickly & i can only say they looked like something in the water was affecting them. i moved the surviving fry back into the breeder box but sadly they'd all died by this morning. i feel terrible have i done something really stupid i tested the little tanks water & it was showing a hint of nitrite (i just used our dipstick test quickly instead of our other test) the test at the same time showed 0 nitrite in our main tank where the little tanks water came from.
where did this nitrite come from i get the feeling ive done something v dumb but i didnt expect to get any nitrite so quickly with teeny fish in the new tank. just before the fish started looking unhappy i had added a couple of other rocks & the gravel had stirred up a little. these rocks had been in our main tank a while ago but had been sitting in the tank cleaning bucket since. did i do something when i added these???i always wash my hands carefully & rinse rinse rinse before sticking them in a tank but maybe i contaminated the water with something???
we've had our main tank for over 3 years & i thought that i understood cycling etc but now im not sure.
has anyone got any ideas???
i feel really bad as it seems like its something ive done to kill the little ones, i know theyre quite delicate & dont want to make the same mistake again.
thanks

 

[afireinside]

hmm, did u cycle teh tank? that could be a problem. plus, molly babies can get stressed pretty easy.

 

[Tokis-Phoenix]

Did you acclimatise the fry to the new tank and for how long?

Although adult livebearers are very hardy and can survive cycling tank conditions most of the time, livebearer fry are very sensitive to water stats like any baby fish and cannot tolerate ammonia or nitrites.
Your tank was showing nitrites indicating it was cycling, despite cloning the tank like you did, you should have taken precautions and made sure it didn't mini cycle as they can do for some days.
It was an easy mistake to make, don't blame yourself but just remember to take more precautions when setting up a newly cloned tank- mollys like other livebearers can produce alot of fry on a monthly basis so it should be too long before you get more fry.
Im terible at tank dimensions, you should try to figure out how many gallons/litres the tank holds(UK or US) as because mollys can produce over a hundred fry at times, the advised minimum molly fry tank size is 10gals.

 

[millym]

thanks
i think it was a bit stupid what we did & i do feel bad that basically we killed the babies but im now doing a fishless cycle on our little tank so hopefully by the next batch of babies it will be safe & i wont move the babies from the breeder box till im sure.