The CPD have to be one of the most beautiful fish I have seen. BUT they hide ALL the time. At least 5 keepers I know have the same complaint. There is supposed to be a dither fish for them but I will be darned if I can remember what it was - only brushed past that tid bit once and can't remember where either.
At one time I had 18 and I would only see a glimpse of them maybe 2, sometimes 3 at a time in the tank, they stayed hidden all the time if the lights were on. And even when there were floating stems on the top of the tank, to give cover from the light.
If the lights were off they would swim around, but it would be too dim to see them and appreciate their beauty. As soon as I got up to get a closer look it was like there were NO fish in the tank. So frustrating to only see them as dark shadows.
My favorite right now for activity and color is the nano fish Microdevario Kubotai
here are a couple of my pics, not that good but see the violet blue???
Here is a nice bit of info about them
A couple more options might be the Green Neon a beautiful and under appreciated fish. And the already mentioned the Cardinal (if soft water) or Neon (7+pH) - these are often found on sale and cheap to get a large school say 30-50.
Additional considerations... when selecting your fish for the conditions of your water and compatibility are their shapes, and where they tend to hang out in the tank: bottom, mid or top 1/3.
If you have 3 shoals of all the same torpedo shape to size - say Neon, Green Kubotai, and Ruby Tetras, the look of the tank is less interesting than say, Green Kubotai, Purple Harlequin Rasboras, a group of Congo or Lemon Tetras - all differing shapes and colors which look nice together (only as an example of color and shapes not necessarily a specific combo recommendation).
There are so many amazing fish that are offered in specialty shops and online that you might be researching options for awhile!