Ammonia and nitrite are toxic for fish. Biology and your filter should normally be able to keep the values within the problem-free range. This biological balance can be disturbed by medications, overfeeding and/or too many fish and the toxic substances listed can then occur in dangerous concentrations. Immediate action is urgently required to prevent fish loss! The toxic substances are reliably and quickly neutralised by adding JBL Detoxol and rendered no longer dangerous. In addition you need to find out the reason for the formation of the toxins and how to remedy the problem.