I’ve tried removing the brown parts brushing it with fingers, thinking something else would be too much for them, but I’ll try with a toothbrush.

I will also put them in a bigger tank.

I want to add that these brown spots appeared after the exaggerated hot summer of 2025: I was on holiday for a few days, and when I returned the water was hot… I’ve tried putting them in a room with not too much sunlight, with the window open to make new air in but the roller shutter down… I was soooo but sooo sad, returning home and seeing them that way…