Setting up a new tank, 40 breeder, sponge filter, fully planted with mature plants/rocks/wood from my other tanks, allowed to cycle a month with about 200 danio fry. All of the danios were fine for the month and have been removed to grow-out tanks, and parameters are testing perfect (pH 6.5, GH 5, KH 0, Ammonia/Nitrites/Nitrates 0 (was dosing ferts for the first two weeks, but then stopped - the tank is so densely planted that even the nitrates levels are at 0).
I stocked with 20 PFR shrimp (drip acclimated from another tank sitting 2 inches away with the same parameters except nitrates 10) and they were all dead the next day except for 1 which was hanging out on a piece of driftwood for 2 days. After 3 days I found him dead on the bottom. Unsure as to what the problem could be, I did 2 100% water changes and dumped 3 shrimp in the tank to see if it was just a 1 time problem. Same thing - even down to one of them sitting on the driftwood fine for a day before dying.
The substrate is pool filter sand mixed with play sand. I don't believe it could be responsible for these deaths, but I don't know what else could be causing it. I did a small toxicity test with a 1 gallon bucket and a single shrimp...he was fine after 3 days...although I'm a bit concerned that the filter sand in the tank was not rinsed as well as the sand in the bucket. Still, it seems like the water changes should have taken care of that, no?
The other thing that comes to mind is the filter, or fert dosing: it was brand new and I've only ever used DIY sponge filters before, never a commercial solution. If there was something on the filter or in the water, though, why would the shrimp be fine on the driftwood? And shouldn't the 2 100% water changes have cleared out anything in the tank?
This isn't a "biofilm" (shrimp don't die instantly due to that) or "uncycled tank" issue (parameters are fine). My tap water isn't toxic to shrimp (there's a thriving colony tank sitting right next to this one, and I have no idea what could be causing this.
Anyone have any suggestions? I was hoping to move my CRSs into this tank, but I'm not going to risk that until I get this issue cleared up? I'd rather not take down this whole tank and start over from scratch without know what the problem is, since odds are it will just come back.
The only other piece of information that could be helpful is that the shrimp seemed to be lying on their sides, twitching, for a couple hours before dying.
Sorry for the long post. People always complain about lack of information on these sorts of issues
Any advice would be great, I don't think I've ever heard of this happening before.
EDIT: OK, after going through everything it turns out the danios were just eating the shrimp every time they molted. I guess danios can learn to eat shrimp