Help! Harlequin's dropping like flies!

Help!
Three of my six Harlequin Rasboras have died in the last 12 hours. The rest of the fish seem to being doing fine. I've had the Harlequins for a month or two.

Temp 78-79 degrees
Water changes 20-25% every two weeks
Nitrates 0 ppm

30G
5 Julii Corys
1 Platy
6 Neons
4 Red eyed tetras
6 Harlequins (until yesterday!)

Is it possible that they aren't getting enough to eat? I usually feed flakes or frozen bloodworms once a day, usually gone in about 3 minutes, and then I usually drop in 3 or 4 Wardley shrimp pellets each night for the corys. The tetras and the platy will usually also join in the buffet, but the neons and the Harlequins either aren't interested or are too afraid of the bigger fish.

Any suggestions?

Sorry.

Ammonia < 0.25 ppm
Nitrite < 0.05 ppm

Over the past two months, I've added 5 neons and 2 Harlequins. Nothing for the past 3-4 weeks. The rest of the fish have been there for many months, years.

Also, this may be a dumb question, but why is it bad if nitrates are zero?

I use a nitrate indicator that reads from 0-160 ppm. When I say its 0, I guess what I mean to say is its less than 5ppm. I did a water change less than a week ago.

Since I last logged on, I've lost a neon. Very weird, and I'm pretty bummed! Any ideas much appreciated.