Is my axolotl sick??

So a little over a month ago I bought two axolotls. Both leucistic (I believe) one with very bright pink gills (El) and one with whitish gills (Leo). When I bought them they were the same size, and both already had all four legs. Both seem fairly happy in their tank, and they both eat, although El eats much more than Leo. Well it's been a month and El is almost double the size of Leo! I'm scared if she gets any bigger I'll have to move Leo to a different take to prevent him from getting eaten! Is Leo just a late bloomer or could he be sick? Or could this be a gender thing (they aren't big enough yet for me to actually tell gender). They both seem to be eating and neither of their gills seem flipped back, and I haven't noticed any slime on either. My tank is usually 66 degrees and their Ammonia and Nitrite seem to cycle normally, although my PH is still high at 7.6. Please help! Is Leo sick? And should I move him to a different tank?

You pH is normal and as long as it stays constant you should be fine. If the smaller axolotl is that much smaller then it might be better to add a divider to the tank at least until they even out. I divided up my smaller guys from the big ones and they still eat and act normally just a few inches shorter then the others. What are the exact ammonia and nitrite readings and how often/much do you do water changes? It might also be one is more sensitive

Three days ago the nitrite read as a 5.0 ppm, and ammonia was at a .25ppm. I did a water change then, to try and get the ammonia to settle. The ammonia came down to about a .15, the nitrite didn't change much, so I did another water change today. Usually the nitrite isn't that high, but I do water changes every week after using a gravel cleaner to siphon and clean the sand I use. I use pellets to feed them. I just didn't know if maybe a diet change would help, or if he may be sick which would stunt growth.

The fact it went up to 5.0ppm means you aren't cycled and that will affect the health of your axolotls. You need to be doing water changes daily and make sure to dechlorinate the water as well. What kind of pellets do you use? It shouldn't just affect one and not the other.

I've been using sinking carnivore pellets. Iv'e never had the nitrite this high before, it's usually under a 2.5. I'll definitely switch to daily water changed until it gets back down.

Ammonia and nitrite should be at 0. They're toxic to axies. NitrAte should be under 40 ppm. I suggest you learn about cycling your tank so your axies won't get harmed during the process.