Illness/Sickness: I need some advice quickly please.

My female axolotl, Eve, seems to have shorter gills then when I first got her. She was about 10 months old when I got her, now she is about a year old. I have a male, Maximus, he's doing fine. My tank water is clear,I clean their feces out as soon as I see any, their tank stays between 60°F-68°F. For the last few weeks their tank is about 64°F, right now its 62°F, they are in a 20gal tall tank. I'm cycling their new 50 gal tank starting tomorrow.
How long should I cycle the tank? How many filters do I need and what's the lowest power filter I can get? I use sand from petsmart as a substrate.
But back to Eve, is she sick, is this a maturity stage as she becomes an adult? I am really concerned,I hope someone can shead some light, I'm really worried about her.

Could you post a picture? I see you're from Oklahoma. I don't think we've had a member on here yet from Oklahoma who has actually had an axolotl; the vast majority are tiger salamanders.

Unfortunately I only have internet through my cell and I don't know how to upload pic from my phone to my posts.
I purchase my axoltls from a private breeder, I assure you they are axolotls. Can you share any information on my questions?

The illness/abnormal behavior could be from poor water conditions. 2 Adults in a 20 gallon tank will foul the water quickly, which would result in a sick Axolotl. Just because the water is clear doesn't mean it's suitable. How frequently do you do water changes, how much do you change, and what type of filtration do you use?

I was doing complete water changes then I read that is bad so I start doing 3/4 water changes every Saturday. I use the filter that came with the 20gal start up tank. I brought a used 50gal tank for them, I plan to clean it out and get it up and running tomorrow. It use to house fish. How long should I let it run before I can put them in it ?
Eve is definitely stress, I brought some test strips and my nitrite and nitrate are really high, I think that may be why she stress but I don't understand what caused the water to spike. I use sand, two fake plants, and a airstone. Am I changing to much water? I stuck them in the icebox, I really hope it helps her. I'm worried and I really wanna do this next tank the right way. Thank God, Maximus seems fine. Advice on all the questions please and thank each one of you for your feedback.