Hello! Fellow Axolotl owner from the UK here, welcome to the site! You've done the right thing asking us
~
First of all, as you're a new keeper I would like to share some information with you. I will try not to blab on, I understand you did a lot of research, but I've learned a lot from handling personally, and made loads of mistakes. I'd dislike you to go through these too!
I keep my axi in a 90 gallon tank, it's 3ft in length- perfect for a single axolotl - however you can get away with keeping two in a 3ft tank. A 10 gallon tank however, is a little too small. If you have limited space, you're best getting a 40 gallon tank at least. Length is way more important than height!
Give your axi sand substrate, and make sure it's fine sand. Please syphon it first, you can't be too careful! Sand is recommended for an Axi who is more than 3 inches long.
Don't crowd your tank, as pretty as you want it to be, less is better especially on the floor. Axi's love to have a good strut about and objects can take up their space. That being said, you're better off having a small hide (like a log), a couple of large pebbles, a couple of plants (fake or real), and maybe a small decoration of your choice. My tank has all of the above, other than a decoration- I just use a moss ball because it's rather cute.
Please keep your tank filtered, this is very important and yet a lot of owners don't do this. It helps keep the tank clean, air to cycle in the water, and saves you from cleaning and replacing the water every day.
Do not buy an aquarium with a heat trap hood. Keep your lid off, and put a mesh over the top to stop your Axi jumping out.
I will get stick for this, but I didn't do the ammonia cycle. My Axi has been nothing but healthy, and I've had no problems in my tank- and I've had it for over a year. I just use water conditioner to get rid of those nasty chemicals and age the water before I put it in the tank.
Now I hope that wasn't too much! Here's the short answers to your questions!
What do you wish you knew as a first time owner/ what should I know?
Don't clean the water out so much. Clean every two weeks, and take about 20% of the water out. Do not put the tank in front of a window or bright light source, or even a radiator. Melanoids and other dark Axolots will get white patches on them, unless it's fuzzy don't worry about it.
How expensive is it to care for them?
Not too bad, just can take a lot of water. You can dig your worms up in your backyard, as long as you don't use harsh chemicals. You can buy them online or at a fishing store, make sure they are only in soil and have no additives. Roughly a year? Waaay less than £250 for me!
What do you feed your axis?
EARTH WORMS ARE THE KING! Unless your Axi is tiny, earth worms are the yummiest and best snack for a healthy growing axi. They keep them super healthy, and they're fun for your axi to wrestle/hunt with
What kind of filter do you prefer?
I use the one my tank came with, I do not know the model but it was by Marina. It's hooked onto the back of the tank, and is a tiny waterfall. Just a warning- they can hum and sometimes wurr a little so if you're easily disturbed do not put the tank in your bedroom.
What is your favorite color variations?
I'm not sure what this means? Axi colours?? I like Lecustic, and Wild
my axi is a dark wild!
Hope this helps