You need to do a couple of things to keep him healthy.
1) Get the pleco into a different tank. If he's a BN you need at lesat ten gallons, preferrably 20. If he's a common pleco, look at 75 gallons. They have a huge bioload and are simply not suitable for a 5 gallon. As well as dirtying the water to a dangerous degree for everybody else, keeping him in there may stunt his growth.
2) The danio also needs a much bigger tank, at least 10 gallons, preferrably 20. They are schooling fish and when kept alone they can stress. Combined with that stress and the stress of not having enough room, he's living with a constant risk of getting sick.
3) The water is a great temperature for your pleco and danio (could do with being maybe a tad warmer for the pleco) but it is way too cold for your betta. It needs to be at least 76, with 78-82 being optimum. That's another reason why his current tankmates aren't great.
It would really help if you could fill in the form that Tikibirds posted. It would give us a lot more to work with.