Hello there! I'm not the most knowledgeable person when it comes to axolotls but because of people on this forum and through reading I'm pretty sure I have the basics down. Before you get the axolotls your tank should be cycled. I don't know much about cycling yet so i would read up on it more but this is the general process. This can take anywhere from a few weeks to a few months. But this insures that your creatures will be safe in there water. I got my axie a little over a week ago as a gift so i didn't have time to cycle the tank before hand so i have to put in extra work for a bit, but I'm willing to do anything to keep my axie nice and healthy. to cycle your gonna want to get Ammonia, Nitrite, and Nitrate liquid test kits (strips are unreliable) and an ammonia source (bottled ammonia, fishfood, ect..) put however much ammonia it says in the tank. Eventually bacteria will grow and eat the ammonia and make nitrite. then after you keep doing this for a while bacteria will grow and eat the Nitrate and make Nitrite. This is what you get out through water changes. The end result should be 0 ammonia/nitrite and under 40 nitrate in a cycled tank. I wouldn't use any chemicals as axolotls are sensitive to almost any chemical. I believe there is a place you can check what is safe for them though. you can buy dechlorinator drops at the pet store.Dechlorinate the water before putting it in the tank to avoid killing any benifical bacteria because if they die you may have to restart your cycle. These should be safe for your axies. If you get two axies you'll wanna make sure there old enough to be out of their cannibalistic stage. other than that a 20 gallon should be enough. The general rule of thumb is 10 gallons for each axie. A filter should be fine as long as it has adjustable flow rate. keep it on the lowest setting. My filter media is carbon. you can also block the flow. I'm not sure about the planted fish tank though, I think it would be fine but I may be wrong. Other than that just make sure to keep the tank temperature between 60-70 degrees Fahrenheit or 16-21 degrees Celsius. Also, an axolotl is a type of amphibian, not a fish
. Good luck with your axies. keep me updated and let me know if you have any further questions.