Freshwater aquarium Nitrate and Nitrite levels are HIGH!?

I have had this aquarium for about 3 months now. i change 75% of the water once to twice a month, and the filter once a month. each time i change the water i stir up the gravel as i get rid of the water, then i use a net to get most of the remaining debris out of the water before adding clean water (which i use conditioner for). i have about 20 gallons of water (in a 50 gallon tank) with two small slider turtles and a beta fish (they get along fine). i have a 20 gallon under water (not gravel) filter (tetra whisper) and a heating element (tube type, tetra also). I have gravel, four plastic decorative plants, and a basking dock. up until the last water change (a few days ago) all water tests were always fine. now my nitrate and nitrite levels are dangerously high, but the ammonia level is normal, says no stress. i recently added the thermometer to the aquarium. so could it be temp related? or what?