Hi, I wonder if anyone can help. I am new to fishkeeping (4 months). I have a 70 Litre tank with 4 Adult Platys, 2 Juvenile (raised in the tank), 3 Trilineatus Cory Catfish and 3 Assassin Snails. Over the last 2-3 weeks I have lost 2 Corys (originally had 5, now 3) and 2 platys to an unknown...