Few questions about two tiger salamanders

i have a couple questions about my tiger salamanders, heres the background story about them, my mom bought both of them from the mall on christmas, my sister took one and i took one, mine changed, was eating well, very social little guy, my sisters wasnt doing well so i took it and got it eating great, he was very scared of me and now he comes over and begs for food, they both were eating great and looked very healthy. mine, hes always been bigger and way more social, he hasnt eaten in over two weeks now, before he would attack the food so fast, i fed him nightcrawlers most of the time and mealworms sometimes, he acts really weird now, he doesnt really move at all, and i have offered him food everyday, i even put him in the critter keeper with a towel over it and he still didnt eat, his eyes dont look cloudy and i dont see anything wrong with him, nothing in the tank has changed, its a 20 gallon long, with coco husk, temps and humidity are good. the other one seems to be bothering him, if it comes over near him he freaks out, he starts thrashing around, before they would always be in the same hide and never seemed to bother each other, just wondering if there is a problem i dont see? should i seperate them? i think the little one is mean now, when i go to feed them( i use tongs) he will snap at mine to keep him away...i know they can go awhile without eating but he has never acted this way before...and also, how often should i clean the tank? i never see any poo or smell anything bad, the water is usually pretty clean, but i clean the water bowl every day...i have 100% coco husk, i used to do half top soil half coco husk, but i didnt have any top soil last time i cleaned it out, could this be a problem? thanks!

Using coco-fiber alone is not a problem as long as you maintain a moist environment. Are you using coco-fiber blocks that need to be reconstituted with water, e.g., Bed-A-Beast? These work well either as a sole substrate or mixed with soil. I saw a picture you had posted previously and the substrate looked dry...might have just been the way the photo was taken however.

You mention humidity - what is it at? You want a moist substrate with good ventilation...not stangnant moist air.

Re frequency of cleaning the tank, I clean and change substrate every 5-6 weeks when using coco-fiber alone, less frequently when mixed with soil.

Re not eating, this could be of concern or not...depends on why he is not eating. If there is aggressive feeding behavior, I would separate the two. Nipping accidents can occur and lead to infection.

Assure that your husbandry is appropriate. There are two caresheets for tiger salamanders in the Caudata Culture section of the forum that you may want to review if you have not already done so.