Seems many water suppliers are "modernising" by switching to chloramine for disinfection rather than chlorine. Chloramine brings several issues:Well, apparently our tap water is running high on ammonia currently. The tank sample tested 0 for ammonia so it's been processed already but I'm guessing the water change caused a slight spike which was just enough to upset the shrimp?
- chloramine contains ammonia, so a big water change can mean a big ammonia spike
- not all dechlorinators remove chloramine - check the label carefully!
- some cheaper RO systems (e.g. RO Buddie) do not remove chloramines - you need to add an additional stage to the filtration
Not sure if this is in anyway related to your issues, but worth bearing in mind. You should be able to check on your water supplier's website and view a water analysis to see if chloramines are being used. Dechlorinators like Seachem Prime remove the chloramine and detoxify the resulting ammonia for 24-48 hours which is hopefully enough time for your bio filter to process it. But in a shrimp only tank with low-level biofiltration, it may take longer to process after a big water change and hence the ammonia can re-toxify once the Prime wears off.
Seachem make a free ammonia alert thingy that stick on the glass inside your tank and lasts for 6 months. Something like that might be worth looking into?