That is a very nice sized tank for a pair of axolotls - plenty of room.:happy:
I would say you've been lucky up to now. Very often when people don't understand the cycle, their first axolotl post is about a seriously sick axolotl, not one with a nipped tail and a bit of fungus. And just looking at the tail, I'm guessing that your axolotl is otherwise healthy. (Can you post pictures of both axolotls?)
So, here is a simple explanation of the cycle and the beneficial bacteria and all that. This is especially important since you are getting a new filter - your luck may run out if you don't understand the process.
Ammonia (produced by axolotls and decaying organic matter) is toxic. When you put your axolotl into the new tank, ammonia starts building up in the water. If it builds up too high, it will make you axolotl sick or cause ammonia burns.
Bacteria that eat ammonia will colonize the surfaces of your aquarium and decorations, etc. It takes a couple of weeks. Unfortunately, these bacteria eat the ammonia and then produce nitrite. Nitrite is also toxic and will make your axolotl sick if it builds up too much.
Bacteria that eat nitrite will also colonize the aquarium, and it also takes a couple more weeks. They produce nitrate, which is less toxic. Too much nitrate will stress your axolotl, and is associated with fungus outbreaks.
The nitrate is removed by water changes. It is also removed by plants, but that doesn't eliminate the need for water changes.
This growth of good bacteria is called "the cycle", and if they die for whatever reason, they say "the cycle crashed."
If you started with a large tank and a very small axolotl or two, your tank may have cycled without a huge ammonia or nitrite spike, but that is not typical. I think this is where you got lucky - that is a big aquarium.
Filters provide much more surface area for good bacteria to grow on and a steady flow of water to the bacteria. ==> If you take your filter out of the tank and replace it with a new one, there may not be enough bacteria left on the other surfaces of the tank to keep the ammonia and nitrite from spiking. Your axolotls are bigger now, so they produce more ammonia. So you need to test the tank parameters daily for a while, and do water changes if the nitrite and ammonia levels get too high.
==> The beneficial bacteria are why you never want to sterilize everything (with rare exceptions). Even changing water with undechlorinated water can kill them and cause an ammonia/nitrite spike.
Issues I see:
1. If I computed properly, you have at least a 170 L tank. (I suggest getting exact measurements and getting online to compute it.) So, you probably still need a bigger filter. Or you can run both filters in the tank (which I recommend for now anyhow, if the old filter hasn't already been take out and dried out.) A rule of thumb is you want the filter to have a flow rate of 4 times the volume of the tank per hour. So for a 170 L tank, you want 680 L/hr.
2. 5 L a week isn't enough. Most people recommend a 25% water change every week for axolotls, but your tank is large for two axolotls, so you can probably get away with a smaller change, but not that small. However, ==> your nitrates are probably too high, due to the smaller water changes and that may be making the fungus worse. You need to keep the nitrates under 40 PPM.
3. It is best to stick with tap water (properly dechlorinated) as long as there is nothing wrong with it. It is easier to get and so it is easier to do large enough water changes. (If you do use bottled water, make sure it isn't distilled or reverse osmosis - axolotls need the minerals).
4. Test strips are notorious for being inaccurate. The liquid test kits cost more upfront, but are more accurate and last longer, making them cheaper in the long run.
Things to do:
1. You probably need to do some rather large water changes to get the nitrate down, but you should test first to figure out how much it needs to come down. If you don't have a test kit yet, change 25-50% a day for several days.
2. Since you have a new filter, you need to monitor the ammonia and nitrite daily to make sure it doesn't spike, and do large water changes if it does.
3. Watch the tail, and if the fungus starts getting worse or spreading, you will want to try a salt batch or tea bath. Some people swear by tea baths - more gentle than salt, apparently. I don't thing your axolotl needs friging, so you would do a salt bath with tank-temperature water. A salt bath tutorial is here
http://www.caudata.org/forum/f46-be...axolotl/72698-salt-bath-picture-tutorial.html Search the sick-axolotl forum for "tea bath" and see what people recommend.