Strange, I've never seen wigglers all huddled like that. Lack of sufficient oxygenation maybe? I keep my TDS around 80- 100ppm when raising fry, keeping things as steady as possible for the first month. I do plenty changing of water, just keeping the water consistent. In the wild rams come from water as soft as < 30 TDS.
I prefer to leave the fry with parents until they learn not to eat them, and typically remove them from their 8gal breeder tank when the parents are ready for a new spawn, which poses risk to the now 10-14 day old fry. I siphon the fry out with
airline tubing, or sometimes move the parents to another tank, raising the fry in their original mature tank (had more success that way) until they're 'cramped' and they go into a bigger grow out tank.
When removing the fry instead:
I had fry gradually dying off on my first few attempts, but rectified that with having them in a larger volume of water - a +-15l plastic tub, with small
sponge filter, airstone and a few pieces of horntwort/ susswassertang and a few
salvinia bits. I feed crushed egg yolk for first 3 days of free swimming, as well as
hikari first bites + banana or microworms (or vinegar eels, but they are less nutritious), by squeezing the mix and some water at the fry (in a small dropper thing, like a miniature turkey baster). I do this from the time they are wigglers (minus the egg yolk) so the parents realise there's food around.
After a few days, I add baby brine shrimp to the feeding routine. By now one can see them eating pretty clearly (magnifying glass also useful). I run the lights as normal, around 10-12 hours a day. Large water changes ever 2 days (50-60%) or 30-40% daily. The hardest part IMO is finding the right balance feeding versus water changes (ie. overall quality). I just check the TDS during this time, for every W/C and in general. So long as your tap/ RO mix is consistent, there should not be issues.
Two weeks in and you're usually okay, more so after a month. My pH ends up around 7.5 but I don't really test it much, and instead just stick to routine.
Some links that may help:
(good watch)
If you get really lucky, mature pairs will raise generation after generation without eating them, in the same tank.
This guy managed it:
