The substrate is a mix of crushed coral & ls. I don't have MH but it does worry me that you ask(what in the tank am i killing?) I have 110w of pc. A custom skimmer powered by a rio 600 and a CAP 1200 return. 40lbs of florida aquacultured lr. Use only RO water I get from the grocery store(a tip I picked up from the board) and I add kent iodine, calcium, & Stronium/moly. The clam has been fine so far. I know the tank is young, but it was a great deal at a lfs. Let me know what I'm doing wrong, or if there are any suggestions.