Sounds like you've overstocked the tank a bit too early bronty.
I presume you have a filter for the tank?
Also you ought to read up on the nitrate cycle, its basic but useful, how ammonia is converted to nitrite and then nitrate. Early on in the tanks life Ammonia will be a problem. As the filter matures it will gradually speed up in ts conversion of Ammonia to nitrites and then nitrates.
You can get little test kts to tell if you have too much ammonia, nitrite etc int he water.
Have a read through this article to give you some good guidance
http://www.practicalfishkeeping.co.uk/pfk/page s/show_article.php?article_id=361