I am trying to get the new axolotl achievement and I just can't get them to spawn.  I have tried making water caves just below the surface, 10x10, then blocking the entrance with dirt so there is 0 light.  The floor is entirely natural stones.  I spawn glow squid a plenty.  But no axolotl.  I even found a lush cave while mining and built a pond there (oddly it didn't have its own).  Can't give the size cuz I worked with the natural contours of the cave.  Again, glow squid but no axolotl.  I have experimental features turned on.  In the lush cave world I have only used peaceful mode (the cave is large and open and I'm not that into hoards of the undead).  In the other worlds I've also tried easy mode.  Either they spawn rarely or they're extinct in my worlds...