The 10 new fish will cause the ammonia in your tan to start building faster. You are not trying to get zero nitrates. Zero nitrates means that you are controlling the water completely with water changes. That doesn't seem so bad until you realize that the more fish you have the bigger and more frequent those changes will become. That can quickly become over 50% daily if you fail to properly build the bacterial colonies in the filter. It actually is starting to make me suspect that someone talked you into putting zeolite into your filter. If you did, the chemistry will be just fine until the day that the zeolite is exhausted. On that day, the ammonia spike will kill lots of fish before you notice it. The stuff works great to control chemistry but is very, very unforgiving of you failing to test the water very frequently.
The real answer is no more fish, that means none at all, until you start getting the chemistry of your tank to respond properly to the ammonia that is coming from your fish. As long as there is no nitrate and no other chemicals in the water, you are changing the water often enough for the fish but are not building bacteria. When you are keeping the ammonia and nitrites near zero but seeing the nitrates building slowly, you are nearing a cycled tank that you won't need to fight constantly. In my well established tanks, I don't worry about my fish if I miss a weekly water change because I know the bacterial colony will take care of the poisons in the water. In a new tank without those bacteria, you can't let a day go by without testing, just in case you have a big chemical spike that day.
The real answer is no more fish, that means none at all, until you start getting the chemistry of your tank to respond properly to the ammonia that is coming from your fish. As long as there is no nitrate and no other chemicals in the water, you are changing the water often enough for the fish but are not building bacteria. When you are keeping the ammonia and nitrites near zero but seeing the nitrates building slowly, you are nearing a cycled tank that you won't need to fight constantly. In my well established tanks, I don't worry about my fish if I miss a weekly water change because I know the bacterial colony will take care of the poisons in the water. In a new tank without those bacteria, you can't let a day go by without testing, just in case you have a big chemical spike that day.
