Zebra Dario are pretty sturdy fish but try to keep both at .25 or less. Always have some change water setting in bucket ready to go at proper or at least room temp.
The fact is even though fish are eating all food they poop it right back out in a form that actually raises ammonia levels quicker than just letting food biodegrade on it’s own, so ammonia will spike and subsequently nitrite. Do small water changes constantly, enough to keep ammonia and nitrite in check and keep up with stability dosing.
Also ammonium is main ammonia compound when PH is 6.8 or below, when PH shifts above that it converts to ammonia which is more lethal to livestock, so check PH of your change water. If ph out of tap is high let it degas for 24hrs setting bucket and test ph again.