Lava Rocks - Poisonous to Sulawesi Shrimps?

As luck would have it, my recent batch of Sulawesi shrimps started dying again after a few weeks. At first, they look to be quite stable but all of a sudden, they started dying again ... up to 2-3 shrimps a day. :( I was determined to save them this time.

I initially thought it was my water conditioner as I got the cheap water conditioner for all of my tanks. After changing the water conditioner to Tension Gon, the problem still persisted. Luckily, J-wing suggested that it could potentially be the lava rocks that I put into the tank as the lava rocks could be leeching some poisonous metals like copper or sulfur. Since I had nothing more to loose, I decided to remove all of the lava rocks and amazingly, the deaths stopped. However, it stopped a little too late as it already dealt a great blow to my Sulawesi shrimp population. I've also reduced the number of ramshorn snails in the tank so that the bio load remains low.

I've also since started to ensure that the top up water parameters is matching as closely to that of the tank. I've setup a bucket with coral chips in it and have the water in that bucket treated for 1 day with Mosura Shizhen before adding it into the tank to top up the evaporated water. It has been going well for the past few days now and I hope the situation continues to stay that way ...