Shrimp dying every day for weeks

[TotalNewb]

Hello, 

Please help

I have a bay of 9 tanks with different cardina (CRS, CBS, Tangerines Tigers, Super Tigers, Pintos, Blue Bolts, King Kongs) in each, all run off a sump. It is roughly 450L. It has been set up for over 2 years. 

However, in the past few months I have been loosing at least 1 shrimp every day if not 2. 

I got a shipment in a few months ago with some shrimp that were too old to ship and they started dying off - now have only 1 of them left, I had thought they brought in disease but I quarantined them after i noticed it, fed everyone with Fennel, added loads of cinnamon bark and alder cones and did a big water change, after that there were no more death for a few weeks. 

Now they are dropping every day. ?

Parameters:

RO water remineralised as always

Ammonia 0, Nitrite 0, Nitrate 5-10 depending on water change (20 L every week, 40L every few weeks if Nitrate is creeping up). Have Purigen in the sump also to try help keeping it down. 

KH 0 - tried adding Montmorillite clay to raise it - it did nothing, added some crushed coral to the sump the other day, hoping it will help

GH 6

TDS was at 130 - have slowly been raising it to 170 in the hope of reducing the number of deaths

Temp 24 degrees Celsius#

EDIT: PH 5.8

I used to have the UV filter on but after they all started dying I turned it off to allow more algae to grow. 

 

Any suggestions please? Thanks, your help is very much appriciated

 

EDIT: - It mainly seems to happen to the females after they molt - there has been no breeding for the past 2 months. Lots of plants and hiding spaces there. 

Edited May 23, 2019 by TotalNewb

[jayc]

What's your pH?

in an old mature tank, you can get pH crash with low KH.

 

[TotalNewb]

Hi jayc, sorry - its 5.8 - have Shrimp King substrate in all the tanks

[beanbag]

check for signs of bacterial infection.

Turn UV sterilizer back on.

No reason to try to change water parameters.

[TotalNewb]

@beanbag I will do - thank you! Fingers crossed