Water quality has been good (although I must confess because I had to leave the aquarium at my parents whilst away for a year the quality did suffer somewhat). There's no detectable ammonia, nitrates are low, pH is about 7.6 and temps are 25-28C (depending upon the summer temperature).
The largest Pakistani loach of the bunch wolfs down any food voraciously, I'm doubtful that the four loaches are so fat because of hungry appetites however. Is it possible that the fish are gravid?
My concern is they've been swollen for more than a year now. Can they really carry eggs for that long? Unless of course they're getting continually pregnant and I just can't tell them apart...
Below's a pic of the fattest one:
but to be fair your females look in great shape , as the breeding of fishes like these is not easy all you can do is hope the male finds a way .