Hi there. I am completely new to this. I have been asking the shop at each stage what to do and my tank was ready 3 weeks ago for fish. I put a clown in first and then got another one. The first one was always very quiet and hiding below the filter etc. Last night the first clown fish died and the 2nd has started swimming a bit tilted. When I looked in the unit underneath the water steriliser was not on but hasnt been since the tank was set up (by the shop). What can I do to stop the other one from dying as well?