the fin color can be due to any kind of stress. its not uncommon for the fins to lose some red . also, if its just one fish, its probably not sick.
increase your filtration ASAP. (until you do that, stick to 2 25-30% waterchanges a week)
make sure to feed them daily
and add some kind of hiding places. they like plants, so if you can add some live plants that would be best. driftwood would be good too. the smaller ones will need somewhere to retreat to when they get picked on.