I've recently tried introducing assassin snails to my 75 gallon community tank. But they eventually die after a one or two weeks. Ammonia, nitrates and nitrites are all at 0.0 and the PH is 7.0. The community tank fish are 5 neon tetras, 6 black neon tetras, 4 zebra danios, 6 guppies, 9 lambchop rasboras, 11 pygmy cories, 4 kuhli loaches and some ghost shrimp. I've also got a major infestation of malasian trumpet, pond, and rams horn snails which prompted me to buy the assassin snails. I've purchased 12 assassins from three different stores in the last two months. As of this morning the last two have died. I have had no problems with water quality or stock. The tank is low tech and heavily planted. I do a 15 to 25% water change once a week. The tank has been established for 12 months. Any thoughts?