I cant keep annomina level down in frog tank?

i have a 2.5 tank that i put 4 little and i mean little adf in there, i cant seem to keep the level of ammonia down in my tank every three days its at 2.0 again dark green on the liquid testers, for any that dont know, its bad... all the frogs are alive but i can tell when its high because that get life less.... so whats the prob... i have live plants in there to pramote CO2 and i have a inside whisper filter, is it possible that with such a small micro filter that the carbon is too dirty and needs to be changed more often ....and it needs to be said i did add one little sepae tetra that was nipping in the big tank... but only for time being can one fish throw off the ammonia that fast, he has been in there 2 days with frogs, i do not know if it was bad before the fish because the tank is only 3 weeks old and this is the first time i have HAD to test it due to life lessness, but it has been changed before......