6x2x2 ammonia problems

[G Robbo]

i guys iv had my 6x2x2 tank for about 4 months now, i have 3xsilver dollar and a 30cm jardini.my filtration is 2x hopar 2200 l/h canisters, i do a 25% water change once a week but i cant get my ammonia below .25ppm and by the end of the week its back up to 1.0ppm. my tap water has 0 ammonia, can someone with any ideas help me out thanks Grant