I'm assuming this is a new tank you are trying to cycle and you have another aquarium that has an established and mature biological filtration system. The best thing would have been to stick a new aquarium filter on the established tank and let the nitrifying bacteria establish in there for a few weeks and then use it in the new tank.  If you can transfer some gravel from the mature tank and put it in the new 38 gallon tank, this will help greatly  because there  is a lot of nitrobacter (the nitrite reducing bacteria strain) on the gravel and the nitrite level should drop.  Another thing you can do is borrow the filtration system on the old tank and switch it over to the new 38 gallon tank  for a while ( only do this if you're not trying to quarantine the two shubunkins since you don't want any cross-contamination of diseases) or you can take some biomedia (foam,etc..) from the mature tank and place them in the new 38 gallon tank to give it some additional nitrifying bacteria.  

Live plants are always helpful in reducing the harmful stuff like ammonia, nitrite , and nitirate. Partial water changes should help in reducing nitrite and nitrate also.

The previous speakers are both right.  In my experience, when I've had a nitrite spike, it helps to do a large water change (as already stated by Nossie) and maybe add a product such as Prime, which allows cycling to continue uninterrupted while detoxifying the nitrite, making the water safe for your fish.

Keep a close eye on them as well, and watch for signs such as burns (white patches), flashing, and blood on the body and in fins. I wouldn't add salt just yet unless you get warning flags such as these and need to help your fish. 

Good it all sorted out
How big is the tank and how many fish?
I also wonder how often you're testing the water?