Upgrading! Help!

BrackishTanker

Hello everyone, this is my first post on tropical fish keeping! I came home from a 2 month trip to Europe last week to find my brothers Brackish Figure Eight tank to be a mess. water changes were done... not as much as they should have been though but i believe the parameters are OK the gravel was not getting siphoned and now is covered in algae, i fear moving it would cause major spikes in dangerous bacteria... As i felt bad for the pufferfish i started to do some research and found that 10 gallons is not enough space for him so i have decided to take the challenge into my own hands and upgrade him into a 20 gallon tank! The tank has been running for about a year now and is beyond cycled. I'm wondering what i should do to upgrade the tank! i was told to remove the water from the 10 gallon tank and move it into the new tank then fill the rest with dechlorinated water or better yet, reverse osmosis would match salinity, PH and temperature as to minimize stress levels on present bacteria and my fish. once the tank is filled with the new and old water i would then immediately move my filter and puffer fish over. i was told this was safe because i already have a large colony of bacteria in the tank due to the amount of ammonia made by the puffer... please help me out! any info is greatly appreciated!! thanks again!

BrackishTanker

I will upload pictures ASAP