Can Clay be exposed?

[hypersushi]

I used not to have any substrate in my tub and I kept and bred guppies in there profusely. But recently, I put clay in it and planted Tiger Lily and only one guppy and a few ghost shrimps can survive. The rest will lose their sense of balance after a day and then die shortly. I want to know if the normal dull grey clay will leach some sort of poisonous chemical? I tested ammonia and it is below danger levels.

[mtanec]

what kind of clay?? Garden clay?? you may want to check on PH also.. some may consist rich metalic content and undesirable toxic matter like lead which can be the main cause..