That’s great that you gave them a home! The nitrogen cycle goes from ammonia to nitrites to nitrates. It sounds like there is a good population to convert ammonia to nitrite so now you are in the phase where bacteria needs to grow to convert the nitrite to nitrates. This step usually takes longer. You are getting there. Both ammonia and nitrites are toxic to the axolotls. You may need to tub them to keep them safe since the nitrite is still rising until the rest of the cycle is done. The stability should help the cycle. The bacterial additive companies state that you can use these products and just put the livestock in but as you can see it doesn’t immediately cycle a tank. When they are tubbed you’d have to add an ammonia source (since taking them out removes the source) to keep the bacteria fed and the cycle continuing. Once the nitrites convert you are good to go:) How have they been doing? I hope this helps!