One of my cardinal tetras has become extremely bloated. Or is she full of eggs? The others are fine. She’s always had a larger belly but seems very big the last two days.

40 litre tank
Ammonia 0
Nitrite 0
Nitrate 10.

Fed on hikari micro granules. Frozen daphnia, cyclops and artemia. Sometimes flake. Once or twice a day. Small amount. But hands up I prob over feed.

It's difficult to tell whether it's over-eating or eggs or something else that is causing her to bloat.
I had a female x-ray tetra who was very similar to this - in her case, her expanded size was gradual and remained that way and became more extreme gradually, and I put it down to the vast quantities she ate in comparison with everyone else. Sadly, I think it was her obesity that ultimately killed her.
As your water quality and feeding regime sounds good, working out the cause may come down to how sudden the increase in size was. If it was sudden, then it may be worth treating with Epsom salt bath dips to help draw out any excess fluid. If gradual, then it may be over-eating / skill at retrieving the food, in which case it's difficult to know how to treat, other than to reduce feeding or possibly separating her from the others temporarily to give the others the opportunity to eat but which may stress her unnecessarily.

Hi thanks for replying. It’s definitely crept up over months. So maybe over feeding but she didn’t go for any daphnia last night (gave in case she was constipated or the pea I put in). Spoke to a guy at work today who thinks she’s egg bound and suggest salt bath. So I got some Epsom salts and gave her a 12 minute bath and put her back. Not going to feed tonight though just in case it helps. Although the guy said if she is egg bound and can’t expel them or reabsorb them then it’ll prob kill her eventually. Poor thing. She copes with salt bath well tho.

Thanks again for advice. Really appreciate it.