Ammonia levels

[BeacauseImAnElf]

We upgraded our tank 2 weeks ago. Everything seems happy except the temperature is a bit high.
We added 2 new fish on Wednesday found one of these dead this morning but think it died yesterday.
The pet shop tested our water this morning and said the ammonia was high.
Having googled it all the causes don't seem to apply and apart from the one new fish dying all the others are fine.
Just wondering if any of you could shed any light on what may be causing it.
Just going to do a 50% water change now to see if that helps.
Thanks

What type of fish where they and how many did you move from the old tank to the new?
The way a tank works is good bacteria build up over time which eats away at the ammonia, which is why you can put all your fish in a brand new tank at once as there's not a enough bacteria to cope with all the fish waste. Upgrading tanks is the same as setting up a new one apart from you can move old water, gravel and filter media over to which can help speed up the cycling as it will have some good bacteria on. Ideally you should've done a water test before adding your new fish to check it was all cycled properly. Don't do a 50% water change only ever do 30% as the very max in your case only do 20%!! Other wise you're taking out too much water and therefore the good bacteria and just brining the tank back to the place you started from. Test your water before you add any more fish.

If your tank is over stocked your ammonia is always going to be high , if it's goldfish you have they need 50l each and if they are what you have consider moving them to a pond and get something easier and more manageable like platys! Hope that helps just say if you need more info or need anything explained better :)