Tiger Barb Issue

hi, one of my tiger barb has seen a little fatter than the rest, not sure whether thats counted as bloated or not, but as compared to one of my tiger barb last time that was bloated very much, i wouldnt call this one bloated. it has been observed for two days now, positioning itself at a 45 degree angle facing downwards and seldom moves around. however, it would move when chased by others and it staying at the top part of the water. I dont have equipments to check my water parameters, i just have a simple tank thats about 2 feet, so about 40 gallon big, with 3 green barbs and 2 tiger barbs, 2 mollies, 1 danio, and 1 suckerfish. and for two days one of my barbs just seems to be inactive, i've isolated it already and it seems to breathe more rapidly than the others

can someone tell me what has happened?
i did a 50% water change two days ago, added aquarium salt and anti-chlorine, had my filter running
but my the barb seems to be inactive already
what should i do?

Post a clear pic of the barb

the colour of this barb has been observed to become paler rapidly since this morning

I looked closely ar my barb for a few secs and observed that it stays in one position for quite a long while, then theres a red dot on the anus, so what could possibly be the problem

hi, i have quarantined my barb for more than 2 weeks already or maybe one week, and it seems ok, but the colour hasnt returned, its faeces is abit red, and the eyes sometimes bulge out for a while isit normal