Fry health and prosperity Bristlenose and Peppermint

[death_adder]

Hi long time member that rarely posts my penchant is for bristlenose which I love been breeding for a long time commons, albinos and peppermints (l071). over the last few months I've made concerted efforts to document fry survival rates ect.
1st thing comes to mind is my albino and peppermints seem to have fairly poor survival rates losing alot the commons do alot better but still lose a good 5-10%.
i do different tanks with different foods and setups and mainly test with Peppermint and albino fry. most start of good until the 3 week stage then start losing 1 or 2 a night eventually slowing down to 1 a week then stop losing them mostly when they hit 3cm.

Sometimes with the pepps I'll get some with bent spines. Fry generally look starved when i find them dead.
Foods are constant greens mainly zuchini then in my testing each lot get different foods from my homemade food to repashy super green and wafers both hikari and other brands.
Sometimes the fry don't seem to grow much at all then i find escapes in the sumps that are triple the size obviously feasting on the slimey walls.
I have very soft water can it be to soft for these plecos.
Water tests today
1 dgh
1dkh
ph 6.7
ammonia 0
nitrite 0
nitrate 0
temps 25-26c
Tds 50ish

tanks are mainly barebottom all the usually driftwood ect. have tested with substrates as well still have the same sort of issues.
one test i split 1 batch into 4 tanks which were identical in filtration and setup foods where different all barebottom all with sub adults 40 in each. one tank had 40 survivors after 8 weeks the others had lost 25% and one more than 50%. two of the tanks exhibited spine issues. but upon repeating the same experiment never managed to get the 100% again.

Albinos never have bent spines but seem to die of just as much as pepps.