Yup chin is right,
Its because the importers / suppliers dont want to pay $$ for the better quality fish, and therefore we have to settle for the crappy ones.
simple as that.
I would suggest adding in more discus, they swim in a shoal, so another 4 would be good, the numbers will make them feel more comfortable and less shy.
The one thats not eating now is probably trying to find food at night when the lights are out.
Feed high protein foods for good growth.
On food packages have a look at the ingredients and check out the protein amount on it.
Anything above 45% is good, theres some foods that are 50%+ but not many.
Like sharn said if they are using their gills irregularly -
A common way of finding out if the discus has gill flukes or not is to watch their breathing, if they are breathing heavily and their gills are flairing out more than they should, or if they have one gill closed and are breathing out from the other gill.
HTH
Jeff
Hmm in your given tank size id have to say that you could get up to 4 or 5 + other fish.
If you can find a pair id say stick with that, if there isnt a pair available...... kribs are easy to pair anyway, most of the time if you put a male and female together that are mature then they will normally pair within 1week.
Are you talking about beach sand or coral sand?
Both should harden the water and raise the ph. Ok to use aslong as ph isnt too high.
Id rather spend a little bit of money and get some fine gravel 2-4mm size.