Dying fish

[KenW]

I had 13 neons and am down to 4. Platy's were up to 9, down to 5. Female betta died last month.
Any ideas as to why?
All the tests results are very good, FW tank is almost 1 year old. They get fed everyday, and eat everything before it hits the bottom. Could they be starving to death? The platys and the betta got dropsy but weeks apart from each other. I just don't know why they are dying off...
any suggestions or ideas would be very appreciated.
Thanks!

 

[mgamer20o0]

what have been the exact numbers of the tank? anything new added?

 

[beglely]

NTD? ive heard it hits fast an hard and is quite merciless with the neons

 

[mgamer20o0]

wouldnt explain why the others are kicking the bucket also.

 

[KenW]

I don't know what "NTD" is. Could you explain?

Ammonia is 0, nitrite 0, nitrate's at about 6, pH 7.6.
Aren't these where they are supposed to be? Am I underfeeding them? I do a 25% pwc once a week. The other fish seem fine: 2 clown loaches, a big pleco, a dwarf angel, a dwarf frog, 2 ramshorn snails, 3 hatchet fish and a catfish. The hatchets are new, got them about a month ago. Everything else I've had since last July and they survived an ammonia surge that killed almost everything else.

 

[drillsar2]

What size tank? Also what test kits you using?