Hum.
I would say ammonia is killing your fish.Fish /fry acclimated to 'poor' conditions often survive[longer] then a new addition that did not have the benefit of growing used to water over time ...
When the ammonia [and nitrites] are gone you should be good..Good luck with the g fish tank !
On to water .
My tap water is [exactly ] ;
PH 7.6
GH 9
KH 2-3 [depends on how you count drops ?]...
TDS 350 ppm..
If you enjoy the chem aspect of fish keeping you are wasting your time measuring anything but TDS more then once as a baseline...
The TDS is really all that matters IMO..
I keep and breed fish that require both 'soft' water [German blue,Electric Blue ,Black Rams ] and fish that require 'harder' water like yours [ swordtails and red picta].
First I will say raising all my fish is done in straight tap NEVER messed with.....NEVER.
It is fine and trying to chase numbers is nothing but a death sentence for your fish not done properly and consistently...
Now for breeding these fish I make water better for them ...Chemistry !
I use RO and remineralize my water for the rams seeking a TDS of 100 or so...Not the softest by far but it works like clockwork...I test NOTHING else.
For my beloved swordtails I use WONDER SHELLS added to my tank and allowed to completely dissolve over however long it takes ..They say it 'sweetens' the water..I always assumed it made the water harder... I guess one could say it does as my TDS in one tank was well over 1,000 ppm ! BUT the PH was down to 6.8... Further reading indicated this is common and the regular addition of baking soda will help to keep the ph from going to low..I am not super concerned with my PH but don't want it much under 7 for the swords if I can avoid it even with the TDS elevated as it is .It takes very little baking soda to keep steady which can be calculated over time and added to new water during water changes...
You need to learn how your tank holds and treats water..What happens to it after adding [Why I tested ph after my start of wonder shells]..
IMO a TDS meter is way more important them all the other test..The PH is helpful at times ,like with the use of the wonder shells but otherwise 'useless info ' IMO past your 'baseline'
I would not add crushed coral or limestone ,although they are a couple of the correct methods to go about raising your ph/kh/and gh..They will all likely climb with anything you try and some uncontrollably over time...
Get a TDS meter [HM TDS 3 for like $20]...
Personally and even to some 'professional' keepers I would not recommend trying to make water harder for any fish besides African cichlids..The wonder shell and baking soda will do everything you need simply ,accurately and safely..
Your g fish tank needs nothing but water changes every other day.They are versatile fish but the water quality [regardless of parameters like ph/gh/kh] needs to be very clean..They are by nature large waste creators and in small tanks they can be their own worst enemy...
Be careful messing with your water as it is literally messing with your fish which can be a short lived experiment...
I recommend this pen ;
https://www.google.com/url?sa=t&rct...p/B000VTQM70&usg=AOvVaw3LKK5WCoEoFgIxQKM-GWEZ