Dwarf gourami randomly dead

[Dondeed]

Hello I was wondering if someone could help me find a reason for my female gourami randomly passing away. This morning I came to turn on all the lights and she was the same as she always was, alert and swimming around. She originally came with a different Male however he sadly passed away due to sickness about four months ago and she had shown no signs at all of any sickness other than missing a notch in one of her fins. Neither does the younger Male I had her with and he wasn’t aggressive or even that interested in her. About half an hour ago I went to go check on my tanks only to find her floating upside down dead with a puncture mark near her gills. I’ve checked my parameters and there’s an unusual spike of NO2 so that might be the reason either that or a tumour of sorts. She was always a quieter personality.

(I don’t have an exact measurement kit atm I’m afraid) Tank parameters:
PH 7, ammonia and nitrite 0 and nitrite at 25 which is unusual.
Tank size 45 Litres and has been set up for little over two years
Using an internal filter
Fish contained are minnows, corydoras, rasporas and amano shrimp. There’s only five each of the minnows and rasporas.
Water change last week 75% as usual I do it every fortnight
And I’d had her for around six months with no signs of illness
Nothing added to the tank at all
I feed them flakes and I feed the Gouramis thawed blood worms by tweezers which they take happily.

Thank you for your time.