added 35lbs of gravel to my 55 gallon yesterday,(didn't remove any) and added 5 stems of italian val,water sprite and an ancharis. I had to remove 4 plastic plants but kept the others in there. Today was water day change and tested water before changing and had .25 amm and .25 nitirite,hadn't had readings on those since the tank was finally cycled 5 months ago. So I did a 50% WC instead of my normal 25% and it took care of the readings. What could have caused this ? I feed once a day. Stock is 3 platies,6 cherry barbs and 5 zebra danios.