Brass fittings kill inverts. hobby grade copper tests won't detect lethal levels which are very small around 50ppbillion.
The old sand is a problem too,imo, particularly if you buried it.
Replace the fittings as soon as you can and run some cuprisorb and/or polyfilter. Water changes may help too. Gfo absorbs some free copper too. Ultimatley the free copper in the water ,Cu, will bind to organics and become less toxic. However, if there is a lot in there it could leach back from surfaces and rock for a long time. I'd run gac as well.
Good luck.
PS If you mentioned the brass first ,I think your lfs folks would have been able to figure it out.
The old sand is a problem too,imo, particularly if you buried it.
Replace the fittings as soon as you can and run some cuprisorb and/or polyfilter. Water changes may help too. Gfo absorbs some free copper too. Ultimatley the free copper in the water ,Cu, will bind to organics and become less toxic. However, if there is a lot in there it could leach back from surfaces and rock for a long time. I'd run gac as well.
Good luck.
PS If you mentioned the brass first ,I think your lfs folks would have been able to figure it out.
