Hi, Venus! Welcome to TFK and a new 55g tank! This is going to be a very fun tank for your whole family to enjoy - I know, because I have one! I set up mine in pretty much the same way, transferred from a 29g (and soon to move into a 125 - be careful, tanks are addicting!) ^.^ Thanks for starting a journal to share your experiences along the way! We all learn a great deal from the varied experiences of one-another, and we ALL love watching a new tank grow!!!
I do have to agree with Bored that you're seeing a bit of a mini-cycle in there right now. It shouldn't take long for your parameters to clear up with all the seeded media from your old tank, but the bacterial colonies may be a bit fragile for a time, so you'll want to keep up on testing and do water changes as needed when toxins are present. Growing a tank slowly always makes for the best environments in the end!
I'm curious as to why such a shift in your water hardness from the old tank to the new, sudden shifts like this can be very stressful on the fish, so you want to do your best to keep the Ph/Kh/Gh as stable as possible for them. In my tanks, these numbers match, or are *slightly* lower than the Ph directly from my tap. There are a few reasons why the water in your old tank could be softer, and keeping up a good weekly cleaning/water change routine is one easy way to keep them spot-on!
I'm curious as to where your water parameters stand, both in tank and in tap, if you wouldn't mind posting the numbers for us? Hopefully all will be clear by morning, but I'm a bit concerned by the shift in hardness, as well as the presence of nitrItes in your tap. . .
One last thing I'd like to point out is - after a few weeks, when the tank is ready to accept new fish, the first thing to consider in all the excitement is the ones who live there already. Tetra and Cories both should be kept in shoals - both species will be bolder, brighter, and far more comfortable when kept in a group of their own kind -the more the merrier! Typically, Cories should be kept over a sandy substrate. . . so you may want to do a bit more research into their needs before bringing any more home.
Can't wait to see where this tank goes next!!! Congrats on 55 gallons!!! When do we get pictures!!!?? ^.^