Question: Axoltol not growing/growing SUPER slow?

Okay, so I have two axolotls, a blackish one named Jorge and a white one (black eyed) named Wendy. Supposedly, they have the same parents. I bought them from a teacher who bred all his axolotls in a tank (he had 5 and put all the girls and boys in the same tank, so we aren't sure if they are siblings). I have no idea what gender they are, my friend named them.

This question involves the size of Wendy. Jorge is pretty big right now, and being the same age, I would expect them to be relatively the same size. They are 7 or 8 months old I think (I got them in March, I think they were born in January). Right now, Jorge is about 5 inches, give or take a little bit. The problem is Wendy is about the size the Jorge was when we first got him (he was about an inch longer than Wendy then). So, Jorge is about 5 inches and Wendy is probably about 2.

My question is, why is Wendy growing so slowly? I ended up having to move her from their 20 gallon tank because Jorge started to eat her legs.

If you are curious about their tank set up, heres the info: There are some fake plants (8 I think) which are relatively small. I bought a planter and put that in there for a little shelter. Because the tank is in the basement, I put a heater in the water because the water was really cold whenever I touched it (heater is set at 67 degrees F I think). There is a filter (I think it is a ten-twenty gallon). They have sand on the bottom of their tank.
Since I moved Wendy she is in a smaller tank, no sand, one plant (fake), and another planter for shelter.

It may just be different growth trajectories which could be due to environmental or physiological conditions. Is there anything wrong with Wendy aside from growing slowly? Salamander growth and development can easily be affected by stress, density, etc.

Neither of them are very big for 7 or 8 month old. What are you feeding them and how often?

You don't need a heater. Axies like cold water and heaters can malfunction so I woulldn't risk it. Do you have a thermometer?
Have you tested the water parameters? What are the results?
It also isn't recommended that you put axies under 5 inches long on sand.

I had one axolotl a while back that was very stunted. It was a wild type and he didn't eat as much as my other axolotls, he didn't grow much in fact he stayed at about 4-5 inches for a very very long time. His gills eventually shrunk down to little stubs and he spent a lot more time in his hide than my other axolotls. Despite being in the same water quality, same temperature, my other axolotls were all perfectly healthy and this guy just wasn't up to snuff. He didn't die, although I fully expected him to be dead one day despite my constant efforts to make him eat more and such. Eventually he was donated to a local organization that brings unique exotic animals to schools and libraries and such all around the state. Last I checked he's still alive, still small, still with stubby gills. I really don't think it was anything I was doing wrong as my other axolotls were all in tip top shape and happy as could be, he was just a bit... Special, as they say.