All good advise, the best way to fight ammonia is to cycle your filter, it comes down to the fact you probably have to many fish in the tank or the fish are to big and are creating to much waste for the filter to deal with at the moment, if you do 25% water change every 3 days and you make sure there is no food waste left in tank, and you have the correct amount of fish in tank for the cycling process,, your ammonia levels should be low during the cycling process. I generally would use about 15 small fish about the size of guppies to cycle 400 litres of water, if you can't balance the tank out by having the correct amount of fish to what your filter can process (removing some fish to another tank) then you will just have to keep doing water changes every time your ammonia is to high, using filter start can help and using bacteria balls in your filter will allso help, using fish to cycle your tank is the best way but you must have the right amount of fish in the tank for the filter to be able to cycle without having high levels of ammonia in tank, the quality of your media in the filter will allso determin how fast your filter cycles, that's just what I have learnt from experience I have cycled alot of tanks and currently have 6 large tanks up and running. Good luck..