High nitrates killed my shrimps?

[AVVITT]

Hi guys, my cleaner shrimp died last week and I checked my parameters, ammonia, nitrite were both 0, pH 8, salinity 1.024 but my Nitrate was 40ppm
I read that shrimps can accumulate nitrates in their body which can poison them over time. Today I lost one of my 2 peppermint shrimps so I'm guessing it's the same fate. I've managed to get my nitrates down to 10ppm now and I'm feeding less but I'm guessing the damage may be already done.
Can anyone confirm my theory or do I need to look at something else? I have another peppermint shrimp, an emerald crab, a sail fin, 2 cardinals and 2 clown who all seem perfectly happy.
I'm about to pop to my lfs to get more salt water and to get mine tested as I'm using API tests and from what I've read they're not perfect!

Thanks

 

[Dkeller_nc]

Shrimp can be somewhat delicate, and not just from the standpoint of water quality. In particular, they're "food" to a lot of reef fish and other crustaceans, and they will even fight amongst themselves - to the death. I wouldn't personally think that 40 ppm nitrates would kill shrimp, crabs and other crustaceans - so long as that 40ppm is an accurate reading and it's not actually 120ppm.