Its always a bit complicated doing a big switch over - I assume that you are placing the new tank where the old one is now.
However, it’s probably not as complicated as you are making it out to be...
First off - You don't need to cycle the new tank for a month - in fact you can have the new tank ready immediately if you work hard to preserve and transfer the bacterial filtration and conditions of your 120 into your 300.
Here is what I did when transferring my Satanoperca daemon (a very delicate species) from a 75 to a 125: I moved all the fish and most of the water into a 60 gallon green yard waste barrel (of course never used for yard waste) and added air, filtration, and heat to it. Then I moved the 75 out of the way and the 125 into place. I filled the 125 half way up with tap water treated with a buffer to match the old tanks 6.5 ph and with 1.5 times the regular dose of water conditioner. Next I quickly added the rather stressed fish and all the older water from the bucket with them into the 125. Once they were in I simply moved over all the filtration that had been on the 75 and added additional filtration with it. Over the next few days I added more advanced filtration, but the important step was getting some water movement and healthy bacteria into the new system immediately. I never had any ammonia or nitrite spikes in my new 125 - it was simply the same system as my mature 75 plus a bit more water.
My suggestion:
- Forget about your south american tanks
- set up two large barrels with aeration/filtration and heat, use the water from your 120, so that you preserve the ph and conditions your fish are comfortable in.
- If aggression or shear bioload might be a problem, place the females in one barrel and the male in another.
- empty as much of the 120's water into these barrels, and additional ones if need be, as you can.
- switch the tanks and move over ALL the old filtration systems. Don't worry so much about new filtration systems you want to add unless you will be unable to add them later... I like to get my fish back into a tank as soon as possible.
- Also move over the 120's gravel and decorations.
- fill the 300 1/2 way up with tap water and add african cichlid buffer to up the PH and add lots of water conditioner.
- Drain the barrels into the 300 with a pond pump or similar to fill it the rest of the way up.
- Move the fish into the tank.
- The fish may not like you for this, but you will have preserved most of their water and will have kept their ph and hardness constant. They should make the adjustment relatively easily.
This process should be doable in one day, even with a large tank (if you have help to move it) and it should preserve the bacterial filtration - making cycling unnecessary. Prepare thoroughly, the less time this takes, the happier the fish. Always have people to call for help if you run into trouble.
Other people may do this in other ways,
But this has always worked for me.
Goodluck,
-Sam