When did you setup the tank?
What stock had it before you added the guppy?? (anything else you added???)
Your bacteria should be in your filter, so vacuum cleaning did nothing bad.
And if the PH was the same, if you used something like Prime everything else should have been taken care of.
You also have a small tank which means toxins build up much faster.
A smal volume of water is always much more unstable than bigger tanks.
It is hard to say and honestly just speculation at this point.
I had it to, me first drop of fry, was only 7 and lost 3.
Found out No2 was at 0.3, and the weaker 3 paid the price.
Which was in a 20L tank like you have...
As I say, you don't know, they could be weaker, they could had a birth defect, they could have been sick, etc etc.
From now on I don't go bellow 10 gallon or 45 Liters.
Might put them in a 5 gallon to breed or to grow out.
But no more baby deliveries in small tanks for me.
(never lost a fry again, knocks on wood, yet)
While saying that, I do have a weaker one from me last drop to.
1 thin girl that doesn't seem to get bigger and is always swimming around.
Not just swimming, but swimming like she has to be somewhere, always.
Started yesterday taking her apart and feeding her extra.
Lets hope for the best...
