Yes I know about the fish you lost - well done for doing the water changes as advised.
Now you have the test kit (which I know is expensive but well worth it in the long run and will last a very long time) you will be able to monitor your water and understand more the problems you are having.
In a fully cycled tank, the ammonia and NitrIte should always be 0, anything higher is dangerous for the fish, this is why you have lost the sharks, guppies etc because as you know the tank was going thru another cycle because of the tank strip down and filter washing in tap water. A mistake I konw you wont make again. Always rinse lightly filter media in old tank water taking out during a water change and hopefully you wont see the problem arise in the future.
As I say, 0.25 is still bad for the fish, but over the next couple of days, 30% water changes should get that ammonia back to 0. Then test weekly for a few weeks to ensure the both the ammonia and nitrIte stay at 0 before considering adding any more fish. When you do decide to add to your stock, do so slowly and not too many fish at one time - this will allow the filter bacteria to catch up with the extra waste load produced by the extra fish.
When you have decided which fish you might like to add, post up here and let us know and people here will be able to advise you on the suitability for your tank size and the compatibilty with your existing fish, hopefully avoiding the problems you had with the size and unsuitability of of the sharks.
/www.fishkeeper.co.uk/
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