Hi, here’s just a few things to considering when keeping Nems .
Flow -they like a current for movement if too much they will move to a different spot .
Beware of wave makers and power heads - a new Nem may well move around and end up in one or on one .
Lights - they need a strong light to give them the suns rays to give them energy
Stability with water - if new they may inflate and deflate this is normal at first as they are swapping their water out . Brown stuff being expelled is ok as a one off but not normally day in day out . Your Nem should not be “constantly shrivelled up”and its mouth should be closed tight .
Feeding is not essential if you have good lighting but a small feed of a meaty type once a week /two weeks will help it put on size,keep it as raw and natural as possible.
There’s some great technical info out there on anemones . This isn’t as tech but I learnt the same way .. get one read,read and watch you tube

. I’m no expert and this is only from my experiences and I now keep a cube dedicated to mixed anemones .... all the best and good luck .