Oh yeah, Bala sharks are very temptermental. They were actually some of my starter fish.. which I am so lucky they survived.
When I got them I had no idea about nitrite, nitrate, ammonia, ph, all that. Just through water in the tank and they are good.
But they are a year and a half now... and growing big!
The black ghost knife is a neat fish... Good choice! Do they eat veggies? or strictly carnivore? My bala sharks LVOE zucchini... just tie it to a rock and leave in the tank.. they and the pleco will pick it dry.

Slate is a type of rock. you can get counter tops made of slate now..
Its in a lot of mountains!
That is a place I could get my slate.. next time I take a trip west. Lol..

Slate? on Flickr - Photo Sharing!
There is a picture of it