Axolotl Babies Dying

I apologize, I'm new to this website and not sure where to post this. But I was wondering if anyone can help me, to tell me what I'm doing wrong.
Let me start off by saying that these are first clutch axolotls- I started with 260 completely healthy babies. Raised them to 3 1/2 weeks with no problems, then randomly started having die offs. It started out with a small amount. Maybe 4 a day- then became handfuls. I hatch baby brine shrimp daily, clean everything with hot water, had the babies separated in 7 different tubs (and one glass tank), cleaned water daily, temperature is consistent and below 65.. I'm not sure what has caused it. After all the die offs I even tried switching water and containers, it has made no difference.
This started 3 days ago, the babies seem to be shrinking- which is confusing because they have full orange bellies, I can see them eating. Also, it seems like right before they die their tails point upwards.
I am down to 30 babies. Please any suggestions or help is welcomed. The only ideas I have is bacterial or they weren't meant to make it because they were first clutch.

When I raised mine some died, I had a massive die off when I added a new tub and I think it was because the bin was probably toxic even after I had cleaned it many times and left it outside for a month. So I doubt this helps but maybe it will.

Sorry this is happening. I haven't raised babies so may not be much help, but are you making sure to dechlorinate the water and changing it daily?