not more than 50ppm FOR SURE 15-25 i would think. Not 100% but I have been told it should never be truly 0, in a safe way... your BB takes away the ammonia which you get nitrites from that, then the other bb takes the nitrites out, then nitrates start to form, and there isnt bb to take that waste out... thats where WC's come in, but doing a 25-30% weekly wc wont take them all out but usually keeps them down to a suitable ppm...
Correct, 1 small ray in a 150gal. tank cant sift through all the sand in the tank, so if you have a spot in the tank that doesn't get circulation, or under a piece of wood, the sand will actually for toxic gas pockets there, and and fishwaste or uneaten food will sit there, and sometimes start to rot/mold on the sand and form the gases in the sand.