Unexplained deaths of young Axolotls

I am new to the forum and hoping someone can help me out with suggestions and / or information.

I have a number of young axolotls born about 3 months ago. They have been growing well and look like miniature versions of their parents. Recently however a number of them have unexpectedly died for no apparent reason.

There were about 30 - 35, housed in a 30 gal aquarium in about 8 " of water. Ammonia and nitrite levels are at or close to zero. Water temp is 58 F. The water is filtered by a sponge filter that provides very mild areation and water movement. They are being fed exclusively on freshly hatched brine shrimp well rinsed before feeding. Regular weekly water changes of about 10% are being done.

Approximately one week ago I found one dead in the tank and removed it. The next day 4 or 5 more were dead. A day later same thing. A couple of days passed and then another 5 were dead. I've checked the water parameters again and done about a 30% water change using about a 50/50 mix of aged water (from the adult axolotl tank and fresh dechlorinated water) yet the deaths keep coming. I'm down to about 15 survivors now and don't want to loose any more.

Any help, suggestions or explanation as to what I should do or not do would be very much appreciated. I have about sixty 3 week old axolotls growing in another tank and they seem to be doing well but I don't want the same thing to happen to them as they older.

In addition to the young noted above I have 5 adults, one GFP male, one albino female, one lecuistic female and one lecutistic male and one male melanoid. I've had them for just over a year now and they have tripled their size since I first got them and they continue to do well.

Thanks in advance for your help.

At 3 months old, they probably need larger food than newly hatched brine shrimp. But this may not have anything to do with why they are dieing.

I would recommend taking the rest out of the tank and housing them in tubs or buckets. Do a 100% water change and clean container daily.

Based on the death rate (first one, then more and more), this fits the pattern for an infectious disease, although this certainly isn't the only possible explanation. On the chance that it is infectious, be sure to keep each tank isolated from all others (no splashed water, no wet hands from tank to tank, etc).