Transfering to a larger tank

[Rissa]

Okay so I have a 25l tank established been up for about a year now I have currently updated to a 65l I put my new filter In with my established tank for around 3 weeks while I was waiting for my diamond gravel quartz to come in once it was I rinsed it and so forth. Yesterday I got my temp to about 23-24degrees then put in my filter after letting in run for about 12 hours I put in my 2 BN plecos to help it finish the cycle I slowly added the new tank water into the bag where I had the 2 BN sitting in the side of the tank. After about 20 minutes I removed the fish from the bag and put them in my new tank not letting any of the bag water get in there.
They were fine swimming around and so forth I made sure I was checking the water ammo was about 1ppm and nitrite was .25ppm I done a few water changes and got ammo down to .50ppm and nitrite down to 0ppm I'm still doing water changes.
Well I woke up this morning to find one of my BN dead over by the filter I was so confused to why she died she was about 6-7cm and my small 3-4cm one didn't.
Should I have just added 1 first? Then the other in about a week?
I'm really guttered I saved her from my friends tank which was in one of the worst conditions I've seen.
My smaller BN is still in the tank I've taken the filter from my 25l and put in it the 65l hoping it will help with the bacteria.
I hope someone can help me try to figure out what happened as I dont want it to happen to my other fish.

 

[Rissa]

I also use the API test it, I have ordered a master kit just waiting for it to come in.
Also should I move the surviving BN back into the 25l? I don't want to lose him either

 

[Rissa]

Okay so now my other BN is dead I've tested the water ammo is .50ppm Nitrite is 0ppm PH is around 7.2 temp is still 23-24

 

[Rissa]

I have two danios as I have sold my other ones off should I have tried to cycle with them instead of the BN?