Insufficient info to comment really. What else is in the tank? How are they? Temperature? pH? Hardness? How long has the tank been up?
P. rhomboocellatus is always a bit delicate when first introduced to a tank, it takes a while to settle.
It is just them in the tank, temperature is high 70's low 80's (my room is hot). PH is about 8 (as it would have been at pet store). The water is quite hard at about 120-180p ppm. It has been up about two weeks, I was uninformed and didnt know about cycle when i started it, so ammonia is at zero, nitrites are fairly high, nitrates are moderate.
They seem afraid of me when i approach the tank, which is troublesome as they run and hide when i want to feed them. There are plenty of hiding spots.
Once the cycle is complete, maybe a dither fish of some sort?