Is my axolotl sick?

Hello everyone!
I was hoping I could get some help. I got my first Axolotl this past Monday from a local pet shop. a week before buying him I noticed he wasn't as active as the other axo they had and when I asked why I was told that when they had received him he was beaten up by other tank mates therefore was in "bad health". I have a weak spot for underdogs and wanted to take him home but was told he was on hold till he got a little bigger so I asked them to call me when he was ready. they assured me he was doing much better however I have had him for 3 full days now and he is still very...blah. He eats OK although I don't think he likes his food very much. They have been feeding him Grub pie and other mushy jello worm stuff, I didn't want to shock him by changing his environment and food right away so this I have been giving him the same(but i am totally up to suggestions about getting him switched to something else) Anywho, here are where my big concerns are.. He hides a lot! I know Axolotls don't care much for light so I leave the tank light off as much as possible but even when its dark he hides. He doesn't swim and when does it looks like hes struggling to get off the bottom. He only swims up for a gulp of air and then he just floats back down. and then I noticed this morning that there is a small lump under his front arm... what could that be?! The place I got him from had him in a tank with small reptile bark substrate... is it possible that he could have swallowed some? could that be the cause of all this? I haven't seen any poop in his tank, could he be backed up? If so is there anything i can give him or do to help? I really want him to have a happy and healthy life so anything info would be great! I am new to this site so as soon as I figure out how to upload pictures I will so you can see what i see.

If you have tapatalk press the plus button. Can't really do a whole lot without pictures! How big is he?


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Never heard of the food they have been feeding. You should start on earth worms (Red Wigglers). Earth worms contain the correct nutrients for an Axolotl.
Caudata Culture Articles - Worms

I'm not sure of the consistency of the substrate they used but it doesn't sound right, but you said 'bark' so it could be possible the Axolotl has ingested it which is causing problems internally.
Caudata Culture Articles - Aquarium Substrates

You need to be fully aware of the Nitrogen Cycle, how it functions and effects your Axolotl. It's key to keeping your Axolotl healthy. Read these links fully:
Caudata Culture Articles - Water Quality
Caudata Culture Articles - Cycling

You will need to purchase a liquid testing kit for monitoring the Nitrogen Cycle:
Welcome to API Fishcare: FRESHWATER MASTER TEST KIT
You can purchase these on Amazon.com

Also your temperature is required to be between 16-18 degrees Celsius, this is the optimum temperature for Axolotls.

How big is your tank? What type?