Neocaridina Shrimp Dying, please help

TW: Shrimp death including photos.

I am absolutely devastated we are currently losing loads of neocaridina shrimp and I really want to prevent this ever happening again if I can and save any shrimp if at all possible. I will put as much information as I can. If anyone can think of any reasons why we are getting these deaths, what we can do to help them or prevent this happening again I will be forever grateful.

Background

We started off with a shrimp and snail tank a year and a half ago, they were doing so well, population was booming. Lots of buried shrimp and shrimplets, I don't think I ever saw dead shrimp in that tank.

About 4 months ago we set up a community fish tank where the shrimp were moved to, maybe three or four weeks ago I felt like some shrimp were missing (we had a smaller population of greens and yellows with our hundreds of blues so I could usually pick them out) and that we were seeing fewer babies and pregnant shrimp. I put it down to a bigger tank with fish and more competition for food but the past one to two weeks we have started to see dead shrimp, multiple in a day.

Our set up & care

We have a 130L (47US gallon) long tank, with aquasoil capped with aquarium sand. We have neocaridina shrimp, bamboo shrimp, 10 pygmy corys, 3 hillstream loaches, 10 kubotai raspora and 3 honey gourami. The tank was initially cycled using water and filter media from the old tank as well as bacterial supplementation as we slowly built up the shrimp population starting with only 20.

We have a light that is on from around 8am to 6pm. The tank has an internal filter (fluval U4). The filter media is well established, it gets a very gentle clean in aquarium water every few weeks. Heater (23c), air stone, live plants (floaters, stem plants and ephiphytes).

Our care routine is 20% water changes weekly, weekly bacteria dosing (api stress zyme), tap safe (api coat zyme) when adding the water, plant fertiliser (seachem fertiliser) weekly. All dosed as per the back of the bottle.

Feeding is roughly what can be eaten in 15 minutes every couple of days, for the shrimp we try to vary between algae wafers, shrimp balls, brine shrimp flakes and some botanicals.

We water test with a dropper test kit (api master test kit) weekly before each water change, we have never had ammonia or nitrites. Nitrates sit between 5 and 10ppm. pH reads 6.7 approximately. The water in our area is very soft reading 0 on kH but this has always been the case with this tank. The gH and kH were much higher in the old tank the shrimp were originally in, not sure why this is the case, perhaps the different substrate, more frequent water changes?

Further relevant information to the problem

The deaths don't seem to occur at specific times ie. after feeding, after a water change. I do not see any issues with moulting - no white rings on dead shrimp, normal healthy moults seen in the tank. The shrimp seem to be changing colour, I am not 100% sure if this is before or after death. I have attached photos of the shrimp including some deceased ones, in one photo there is a blue shrimp with a yellow-ish stripe, I am pretty sure this is a adult blue shrimp with a strange discolouration rather than an exciting new variety we have produced because I never saw this shrimp as a shrimplet.

We have not introduced any new stock recently nor have we used any medications. I am super careful about keeping hands chemical-free, we don't use any air freshener or cleaning chemical near the tank.

The fish all appear healthy and are behaving normally.

We visited our LFS who are great for advice, they suggested a calcium block in case of a hardness/moulting issue so that has been added to the tank, the shrimp have shown some interest but not much.

 

Things I that I am considering could be the issue;

 

1.      The change in water hardness between the old and the new tank, despite the shrimp having been moved a while ago, perhaps this is now reaching a critical mass that we are seeing many shrimp deaths?

2.      A new product I only started using in the last month was a different type of tap safe; API coat zyme.

A bacterial infection in the colony caused by one of the following;

3.      Aggressive sand vacuuming - the past few water changes I have perhaps stirred the sand a bit more than usual, could this have released harmful bacteria now infecting the shrimp?

4.      Temperature rise when doing water changes, we were struggling to temperature match and had a rise in temperature of 1 degree to 24C when doing a water change the past couple of weeks, could this have caused a proliferation of bad bacteria?

5.      Over feeding, is it possible to be over feeding enough to cause illness but not enough to show up in water testing?

Conclusion

If anyone has any ideas or suggestions of what to do I would be very grateful. I am so upset, it is all I can think about and I really need some perspective on the issue.

Any insight into how we can look after our shrimp better is really appreciated, I do want to go back to a shrimp-only set up at some point because they were easier to view and appreciate in a smaller, shrimp-specific tank and perhaps this may be the way forward?