Your girl sounds like she has fungus. I think you should put her in the fridge and give saltbaths twice daily.
Check the fridge temperature first, by placing a thermometer in a cup/glass of water and placing in fridge - check in 20minutes.
If the fridge temp is 5C and above, then it's ok to put her in the fridge. The fridge temperature will slow her metabolism down in order to give her body a chance to heal. Fridging my axies over the past year and a half has been an absolute lifesaver/stress relief; especially early on when we first got axies and two of them got quite sick.
You will have to do complete daily waterchanges on her container while in the fridge. You need two foodsafe containers and a bottle for the dechlorinated water. Or if you don't have the space, one foodsafe container and 2 bottles (softdrink/pop bottles will do)
Using the 2 container/1 bottle method!
1. Put dechlorinated water in a bottle/s and one of the containers and put them in the fridge.
2. In the other foodsafe container, put dechlorinated water in and put your girl in.
3. Make sure the water just covers her body so she doesn't flip over or float, which can cause undue stress.
3. place a lid on the container and wrap with a teatowel, then put in the fridge.
NB: No need for holes in the container, as colder water has more oxygen in it than warm water.
The lid will stop her jumping out if she gets startled, the towel is to stop light flashes and temperature fluctuations as fridge door opens through normal useage.
Tomorrow/next day First salt bath,
a) take the spare container out of the fridge
b) add 2-3 teaspoons of aquarium/tonic/sea salt or uniodised table salt (if you don't have any of the former) per litre of dechlorinated water.
c) Take your girl axie out of the fridge and either lift her out with hands or net her out and place her in the salt bath.
d) Cover with lid and place saltbath container she's in back in fridge.
e) Place her container back in fridge, so temperature doesn't warm up
f) If you have an egg timer/oven timer set it for 10minutes
g) When timer goes off, remove her from saltbath and return to her original container, cover and wrap with towel, return to fridge.
h) Empty the saltbath, and give it a wash with a mix of bicarbonate/baking soda and water.
i) Fill with dechlorinated water and return to fridge.
Same day - 2nd saltbath/waterchange:
i) Remove saltbath container, add the salt and give her a 10 minute saltbath as above.
ii) While she's in the fridge, empty her original container, give it a clean with bicarb/baking soda and water.
iii) Remove the bottle of dechlorinated water from fridge and fill up the clean container.
iv) When saltbath is finished, transfer her back to the clean dechlorinated water, cover, wrap and return to fridge.
v) Clean the saltbath container with baking soda and water, refill with dechlorinated water, return to fridge.
vi) Top up the bottle with dechlorinated water, return to fridge.
One thing you should always do before introducing more axies/fish to any tank is to quarantine them for 30days, this will ensure your new one is free of any problems and/or doesn't pass anything onto your current axie and make them sick.
How is the partner doing by the way? Have you had problems keeping your tank temperature below 24C?
I'm not sure how much 50litres is, what are the tank measurements?
Check all your water parameters - ammonia, nitrite and nitrate.
