What are the maximum nitrite level can guppies and platies fish still survive for how long?

I am new to aquariums. I have my new 54 liters aquarium for 17 days in a minute, I used silent cycling using plants (I have 7 plants now with plants substrate underneath the gravel)), I also introduced bacteria. At the day 7 nitrite reading was 0.1 and at time 11 it was around 0.03 according to my shop, and there is a Nitrate reading of ~25 and ammonia is 0, so I was relaxed that it works, so I decided to introduce fish and I took 3 guppies and 3 platies. After the second day one platy feminine died, but i found later that there are 4 baby fish :), so i though she died of birth (is that true or because of ammonia?(Ph horizontal is ~8, I don't have a test kit for ammonia), after another 2 days a manly platy died :(, the nitrite reading was a bit below 0.5 (the lowest reading on my test kit after 0), could it be that this kill it? On that day directly I did about 40% water alteration. Yesterday I moved all the fishes to a clean water external the aquarium because I was afraid that all will die (the nitrite level is between 0.5 and 1), I kept the babies since they are rock-hard to find and catch, but it looks they are doing fine. So what do you advice me to do next? It looks that at hand is not enough bacteria, I am thinking to bring the fish back to the aquarium and do 20% marine changes everyday and hoping that the fish will survive until the bacteria do better work.
Answers:    54 liters and 6 fish all at one time within a new tank. That was the start of the problem. New tank should only have one or two fish added at a time not 6. Nitrates are fine You really need to achieve an ammonia test kit here. PH for these fish is also fine at 8. Usually nitrites at .5 in more cases than not won't eradicate your fish.

You are now housing your fish outside the tank which is nearly fully cycled and have them once again in an uncycled container. Doing this moving places a lot of undue stress on your fish. They should remain within the main tank here. Water change is your friend here.

When you first get your fish did you acclamate them properly? how about when you removed them from the tank?

You need to return your fish to the biggest tank. Test your water and do 25% water change when needed to keep the nitrites under control.

What ever you do do not keep calculation chemicals for cycling.
There is no hard and fast nitrite number. You can tell though whether it's nitrites from the actions of the fish. Nitrite kills by suffocation, affected fish will be at the surface gulping nouns or they will be right where the most oxygenated water in the reservoir is. Often they end up near the bottom near a corner fast working their gills.

More oxygen in the water will let them survive at a superior level. Salt in the water also let's fish withstand superior levels of nitrite, the chloride competes with the nitrite as far as uptake into the fish.

With your tank producing nitrate, I don't believe ammonia would own built up that fast. Fish should look basically paralyzed for awhile before dying from ammonia, perchance laying at the bottom or just floating in a corner or nearly lying in some plant.

One product that is good to own around, especially if a tank isn't totally cycled is Amquel plus. It detoxifies ammonia, nitrite, and nitrate. Note though it doesn't spawn a test kit read zero and it still allows the nitrogen cycle to bring back going. It's quite amazing, you can have a fish in distress and tally it to the water and the fish usually rapidly recover.

Does your municipality use chloramines in the water? If anything, I would almost surmise it sounds more like them. With chloramines you need to use the proper conditioner to make the marine safe. Call your municipal water department to find out.

I suspect that the fish had a problem when you get them. Some things don't show and it's not unusual to have some deaths shortly after getting fish.

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