Duds
Are you grinding up the food now that fry are in the tank, fry can choke if the food is not ground up, it made a big difference for my fry.
Remember to feed some veg for hypancistrus fry each feeding.
And try putting some Indian Almond leaves in the tank, the general idea is one leave for 10G and replace them weekly, the leaves have a natural bacterial property to them.
You'll find some here;
http://www.ta-aquaculture.co.uk/
Try to feed as near to the fry cave as possible, my fry don't venture far from the cave in the beginning.
And try feeding a little micro worm with each feeding.
Are you hoovering your substrate each day, running your fingers through the sand and hoovering, things like that helped me. making sure the substrate is really clean.
I only do this weekly with an adult only tank, when doing a water change, but find it much better doing it daily when fry are in the tank.
Is there anywhere for the fry to hide that the adults can't get into.
I put oyster shells in my tank to help with low kh, but found fry like to hide under them as well, things like that are nice tight hiding places for the fry in.
Pieces of wood are the best for fry to hide in the cracks in it, but a pile of small stones where the fry can squeeze between the gaps, works as well.
If the parents are breeding i don't think there is much wrong with your water.
The only thing i would watch for is, with such a low TDS, you wil have a low KH, probably around 1, which can be a bit unstable if your tank is well stocked.
Daily water changes will help this.
As Gee said, it's finding what works for you, when dealing with your fry.
My daily routine with fry tanks and tanks with adults & fry in it, is;
At tea time, i do a 1/3rd daily water change because of my low kh tap water, in the process hoovering the substrate well, stirring it up to make sure there is nothing trapped in the substrate.
Fill up the tank from a storage tank, doing this daily helps keep low kh water more stable for fry.
And i feed a little freshly hatched baby brine shrimp, but a little micro worms will work just as well, i just happen to be feeding some corys baby brine shrimp at tea time.
Just before lights out for fry tanks, i crumble up a broad bean beside the adults cave, and place some ground up food ontop of the broad beans, and feed a lttle micro worms.
