Shrimp mass die off
Tw: last photos are dead shrimp
I've had a 10 gal for running over a year with cherry shrimp, a school of pygmy cories, and some neon blue rasboras (sundanio axelrodi). Over the past maybe 3 days? I've lost all of my shrimp except 2 of them, which I've since moved to my rejects tank, where all the shrimp have been doing fine.
I probably had 50-100 shrimp, most of which were all bred and born in there, that are now all gone (the snails and scuds have made quick work of them fortunately/unfortunately). I moved 3 weeks ago with no problems and topped off the water recently with one gallon of RO water.
I don't have much tank maintenance since it's heavily planted (fertilized with root tabs), and besides the heater and filter (aqua clear 20) it's been pretty self sustaining. I clean the filter every other month or so, and I top off the water every 2-3 weeks (either ro, or a mix of ro and conditioned tap).
My water parameters have been stable with no nitrites, nitrates, or ammonia since it was initially cycled. Ph has never been below 7.0 and never above 8.2 (gradually lowered with adding tannins). GH is 9 and KH is 5 (previously has been closer to 7/8). There hasn't been any chemicals, cleaning, or something that got into the tank as far as I know.
Does anyone have any idea what could've happened?