Why won't the ammonia go down?

i have a 30 gal tank live plants 1 lrg shbunkin 3 med. 2tailed egg shaped goldfish) 2 tetras,6 snails,i crown beta 1 pleco everyone gets along perfectly, the amonia levels r thru the roof, i have changed 50% of the water added ammo lock and water conditioner, siphoned the gravel numerous times and the levels are still high why?