I wouldn't put any more than 1 adult axolotl in that tank.
'Deep' has nothing to do with how much water it can hold. 19 gallons is 19 gallons, whether it's 12 inches deep or 2. Axolotls require a lot of floor space. Having a tall aquarium is a waste of space when it comes to axies.
Adding piles of gravel is bad. For starters, that gravel is an inappropriate size for axolotls. It can be swallowed, leading to impaction and ultimately death. For two, piles of gravel will just trap debris and take away from water volume.
You've also provided no hides for your axolotl, and have a fairly bright light. This will stress your animal, leading to illness. And get rid of the heater. You won't need it.