HI to all online I am having the most bazaar problem EVER

I have ran my tank for 12-18 months with malawis, recently I purchased 4 new malawis and plants for my tank. Within 24 hours I was loosing fish I ahve lost in total 12 fish My amonia and nitrites are through the roof and cant seem to get them down at all. Over the last 2 weeks ~I have done three 50% and 5-8 25 % water changes and still very high my fish are not dying as ofetn as they were but cant seem to reduce the high amoania and nitrites

ANY ADVICE OUT THERE FOR ME PLEASE???

NEAL