Sudden tank crash

[EmbarrassedSoNameChange]

Hi, it's a long one so please bear with me. Has anyone had a similar experience? If so did you find the cause?
Tank:
Fluval roma 200
1 inch soil substrate capped with 1 inch sand and additional gravel patches and granite pebbles.
3 pieces mopani wood, all been in since day 1.
Fairly well planted, plants thriving (crypts, java fern, moss)
Heater set to 26°c. Thermometer usually says 28°, generally stable.
Until yesterday inhabited by 6 kuhli loaches, 4 horse head loaches, 4 amano shrimp, hundreds of neocaridina, 3 nerites, 15 neons and 10 guppies. All appeared healthy yesterday
Shrimp and snails are given ntlabs calcium supplement once a week. Feeding is every other day and alternates between sinker pellets, dried bloodworm, flake, frozen daphnia (ran out of daphnia so missed a week)
So my tank had been stable for a good 2 years with 2 internal filters, but the impeller was getting sticky on one of them, so I decided to buy a massive canister filter instead. Set it up properly, using the media from my old filter along with the new filter, all seemed fine for 2 weeks or so, checking ammonia, nitrite and nitrate daily and no spikes, so I assumed all was well with the new filter and it had seeded properly. Filter creates plenty of surface agitation
Usual parameters are 0 ammonia 0 nitrite 5 nitrate using API master kit.

This morning I've lost all my guppies and neons, half my shrimp have had failed molts and loaches are hiding under the sand.
Ammonia and nitrite spiked to 1ppm.
All was fine at 10pm last night, they were all resting as usual. This morning they were floating.
I don't have access to any prime right now but I've changed 25% water and dosed API aqua essentials which is similar and the tests are showing 0 again but I know it's only bound, not gone. I can't change more than 25% without stressing the shrimp even more though.
You could smell the death this morning it was horrible 😭

Have you disturbed the substrate at all? It seems odd the tank crashed after two weeks as the filter should have been ok at that point. Are your temperatures actually accurate? Have you verified them with a calibrated thermometer?

I’m sorry this has happened, it’s soul destroying. If you get to any decent pet shop you can get therapeutic fresh water salt that is used in hospital tanks. If you haven’t already, I’d dose the entire tank ( although I don’t keep shrimp so double check that)

Thank you for replying 🙂 The shrimp can't tolerate salt unfortunately. Yeah the temperatures are accurate. I've kept fish for so many years and it's never happened before. I am fairly experienced. My last set up was marine and it never went wrong even though they're so delicate. I'm starting to wonder if it's been contaminated somehow. Maybe a poisoned fly landed in it or something. The sand turns over regularly cos the loaches are in and out of it so I don't think it's that. Come to think of it there's also some hitchhiker snails in there too that also turn over the sand but I don't overfeed so they've never took over. I've had no more deaths since this morning and the API stuff is keeping the ammonia and nitrite undetectable for the time being, fingers crossed.