You do have other options here besides massive water changes which will then be happening a lot.
The very best thing to do is to add bacteria to the tank. In terms of bottled bacteria, Dr. Tim's One and Only or Tetra's Safe Start will both help a lot and fairly fast. Next, finding somebody to give you some media or substrate from a cycled tank will also add bacteria. The more you can get, the better. Basically, this approach solves the problem by adding the very bacteria you are now working to grow.
Next, the cause of the problem in your tank is the fish. they are making the ammonia which turns to nitrite etc.. You should not have added the tetras when you did. So failing to be able to follow the above suggestions, the next best thing is to get the tetras out. The store may take them back for credit. You can tell them you still want them back, but in a few weeks. otherwise maybe you can find a member here or a friend with a tank who can hold them for you. The idea is you will reduce the ammonia being created in the tank.
Because of your pH. you have minimal wiggle room on your ammonia. At .4 ppm in your tank there is .036 ppm of NH3, the toxic form of ammonia. The other part is not a huge deal for the fish for the short term. The red line for the NH3 is .05 ppm. Because I hate trusting test kits and because folks new to fish keeping tend to make mistakes, I would not want to see your ammonia level go higher and I would monitor the fish for signs of distress. If they show any, then you must do at least a 50% water change. If the ammonia rises, you need to do the same.
This does nothing to help with the nitrite side of things. Fortunately, there is a way to block the effect of nitrite inside fish which does not affect the cycling process because it has no effect on the nitrite. One can add chloride at 10 times the concentration of nitrite to do this. And chloride is 2/3 of good old salt, aka sodium chloride.
But here is the thing to all this, using this approach you can get through fish in cycling faster and likely without harming fish. But you need to do the math and a bit of measuring etc. You need to know what to look for in the fish re being in distress. You need to learn a bunch of stuff to pull it off right. The other option with fish in is that you will be changing tons of water for some time with no guarantee that will it prevent any harm to the fish either.
So, if you can not find bacteria, if you do not want to have to learn about a bunch more stuff to do an old fashioned fish in cycle, and if you also are not really enthusiastic about doing lots of big water changes in the coming weeks, your best option is to get all the fish out of the tank and get it cycled without them. A fishless cycle is easier, faster and can not kill any fish if you make a mistake.
