Loosing Panchax And Rainbows

[Numbersix]

Hoping someone may be able to shed some light on a problem in one of my tanks? Over the last few days I have lost a yellow forktail rainbow and 2 rocket panchax. I have a further 2 fisah showing problems. All exhibited the same behaviour in that their tails went very narrow and their swimming became erratic, often swimming in circles or resting vertically. In addition I have also lost 1 half beak over night. All other tetras and rasboras seem to be ok so far.

No new fish have been added for some weeks. Latest additions were 2 amano shrimp 2 weeks ago and some plants this weekend (which were in another tank for 2 days before planting).

tank details are;

Tank size: 60L
pH:7.6
ammonia:0
nitrite:0
nitrate:10
kH:270
gH:500
tank temp:26 C

Fish Symptoms (include full description including lesion, color, location, fish behavior):

Volume and Frequency of water changes: 60L 20% weekly

Chemical Additives or Media in your tank: 0

Tank inhabitants: 4 rummy nose tetra, 4 copper harlequin rasbora, 4 cardinal tetra, 1 ottocinclus, 2 endlers guppies, 2 shadow cats, 1 flying fox, 2 amano shrimp + remaining 1 rocket panchax, 1 redline half beak, 3 yellow forktail rainbows.

Recent additions to your tank (living or decoration): Lilaeopsis nova zealandae x 4

Exposure to chemicals: 0

Hope someone can help?

Rich