You need to do more than 30%, change as much as possible to get that nitrite and ammonia down. Get them both down below 0.25. To get 2 ppm down to .2 ppm, you need a 90% water change. The best way might be to remove virtually all the water leaving just enough for the fish to be able to swim upright, and refill with dechlorinated water at a temperature as near the tank water as you can get it. Then test again after half an hour to let the old and new water mix. If one or both are still at or approaching 0.25, you need to do another water change.
Don't feed the fish for the next couple of days, then feed very lightly every 3rd day until ammonia and nitrite stay at zero.
What did you do, in detail? You appear to have lost some bacteria.
Since you are US based, see if you can locate a product called Dr Tim's One & Only. It's made the man who discovered the species of nitrite eaters that grow in fish tanks and is one of the few products that work, though not instantly. We can't get that in the UK so I don't know what exactly the instructions say for fish-in cycling
Ahh okay, I am going to need to setup a side tank really quick I suppose to heat the water and dechlorinate before adding it in. I will repeat until I get to those levels.
I will also see if I can locate the Dr. Tim's one and only, the nearest store that might have it is probably an hour away.
But anyway here is what I have done since I setup the tank.
Day 0: Clean substrate, put it into the tank. Took some of the artificial plants I had that were not in a tank and rinsed them then placed them in the tank. I setup a HOB filter on the back with nothing in it and my nice fluval heater and let it run overnight.
Day 1: After reading what you said about using my existing media I realized I could probably do that. I went to the store that day to buy lights and a cover for the new tank, while I was there I also purchased a few live plants and the MTS. When I came home I acclimated the MTS to the new tank, and planted the live plants I had. I started transferring the live plants I had in my other tank and only got 1 or 2 before I had to stop to go to an emergency work call.
Day 2: This is the day I bagged up the fish in my GF's tank, I started by moving the fitlration media into my filter on the new tank and moving most of the decor into the new tank as well. Then I bagged the fish, moved them to the new tank and spent the next 30 minutes acclimating them just in case of minor water differences. After this was done I went to my house, filled a bucket half up with tank water, put the filtration media and all of my decor in there. Bagged my fish and then drove back to the new tank. I put the filtration media in the filter, than spent about 45 minutes acclimating these fish. During acclimation I placed the decor from my tank in. Once the fish were successfully acclimated I programmed my lights for when they would turn on/off and left them off for the rest of the day. About 4-6 hours later I fed the fish some flakes, a couple dried shrimp and I dropped two algae wafers into the tank. Then a few hours after that I did the first water test which was:
pH: 7.4
Ammonia: .25ppm
Nitrite: 0ppm
Nitrate:5 ppm
Which seemed normal to me.
Day 3: woke up and put some flakes into the water, I watched as the fish ate to see how long it took them to eat them. I slightly over fed as some of the food dropped down to the bottom of the tank after a minute. However within an hour when I was going to vacuum that up it was gone, presumable eaten by the fish or snails.
Test results this day were:
pH: 7.4
Ammonia: 0.5ppm
Nitrite: 1.0ppm
Nitrates: 10ppm
Now the nitrite levels here did scare me some and so I did not feed the fish this night, however if was late so I did not do a water change.
Day 4: I worked all day, before work I used a small pinch of fish flakes and put it into the tank, it was gone in about 10 seconds. When I came home I was exhausted but realized my guppy was missing, I found him and took him out flushing him down the toilet and passed out. The fish where not fed. It is also very possible the dead guppy was dead all day sitting in the tank.
Day 5: (today) I tested the water this morning after feeding the fish very lightly again, the food was gone in about 10 seconds. That's when I came up with the test results and then posted on the forums.
pH: 7.4 (although it seemed slightly lighter, but was still too dark to be 7.2)
Ammonia: 0.5ppm
Nitrite: 2.0ppm (it could be higher I suppose. It wasn't dark enough for the 5.0ppm mark but it did seem slightly darker than the 2.0ppm mark.
Nitrate: Was still at 10ppm.
Immediately I was worried and I made this post, and went out to buy an aqeuon aquarium water change system. 50 foot hose that hooks up directly to a faucet or outdoor hose. Easily siphons water out and lets you put water back in. I used that and before water went in I calculated I was replacing 14G out of the 55G which is about a 25.5% water change and put 5G of dechlorinator in the water, then as the water was filling up the tank I put 10G of dechlorinator in. Since then I have come back to this post to see what you suggested, I am about to test the water again and do another water change. I am going to unplug my heater so it doesn't get damaged and will report back in about an hour when I've done a large water change.