ok, so it was definatley cycling with fish but has now finished (the ammonia and nitrite dropping down to 0 show this).
I suspect it was the cycle which killed the fish, hopefully the ones which have made it through will be OK from now on.
few tips then...... have a read of the guide to maintenance linked in my sig which should explain how to clean the tank and filter and what you should be doing on a daily, weekly and monthly basis.
don't add anymore fish for at least 2/3 weeks, you need to let the bacteria colony establish, take some time to do research into the species of fish that you have, your fish combination is a bit odd, you need to adjust the numbers and so on and the sharks will eventually outgrow your tank, so do some research and you may need to shuffle stocking around a bit. the link 'guide to stocking lists' in my sig will start you off with how to work out what fish are suitable.
once a few weeks have passed you can add more fish, but only add a couple at a time, after adding fish make sure you test the water every day for a week afterwards, you may see a mini cycle and you'd need to do water changes to sort it out. after you've added fish, assuming it's all stable, leave it another 2/3 weeks before adding any more fish... then add a couple more fish and do the same thing again until you are happy that the tank is stocked.
