Firstly, go here:
www.axolotl.org and read up.
You need to cycle your tank, as Amber said. It's critical to maintain the health of your axolotl that it's environment is as good as it can be.
The main thing to remember is to keep Ammonia as low as possible - 0 being the goal to achieve. Nitrates should also be at 0, and Nitrites at about 30. Those figures indicate a fully cycled, healthy tank. PH should range from 6.5 to 8 (alkaline) - they don't like acidic water.
Water depth is a personal preference. A general rule of thumb is make the water just a little deeper than your axie is long - but there isn't any harm in having a tank filled with water.
Keep in mind though, the following:
1) If you do fill up your tank, make sure you have a lid/hood of some sorts to prevent escape (yes, axies can jump)
2) The larger the water volume, the larger the work load for your filter.
3) The larger the water load, the longer it takes to cool down in hot weather (assuming you don't have a chiller unit)
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Too many plants? There's no such thing! Well, there may be issues if you have a small tank and 90% of it is plant matter. Plants give off oxygen, and also absorb some of the harmful nasties in the water. They also offer a play area and a place to hide.
Metamorphosis generally doesn't happen - the chances are minimal. There has been some research conducted, and some believe that it can be triggered by diet, and/or the presence of iodine and thyroxine in the water. If your axie DOES morph naturally, it's not a burden, it's a blessing! Their life expectancy may drop, as will their regenerative ability, but a natural morph is a wonderful thing, and not something to be upset about at all
If they morph naturally, then it's what the axie wants to do