I second what doug has stated,
With the new tank, make sure the filter you are buying has an output of atleast 6x the tank capacity. so a 4ft 200L will need a minimum 1200lph filter.
I have over 400 fry at any time in my growout & use an internal filter of 1200lph and an ext. canister of 1000lph.
The internal is used mostly for water flow & movement.
set up the 4ft and add 170L water to the tank.
Heat it up & run it so its warm & ready to go.
Clean the filter that you have in the 2ft tank straight into the 4ft tank. make sure you really squeeze all of the good muck out
Let it run for another 2 days & add all of the BN adults you have & platy adults. (acclimatise them over 20 mins)
SLOWLY acclimatise the water in the 2ft tank, Take out a bucket of OLD water, pour it into the 4ft. Get a bucket of water from the 4ft & add to the 2ft SLOWLY!!!... do this atleast 8 times. then empty the majority of the 2ft water back into the 4ft. You have now partially acclimatised the babies to the new tank water....
Catch all of the babies into the bucket. add water from the 2ft (about 1L into the bucket with fry. now add 200ml at a time of 4ft water every 5 minutes until the bucket is 3/4 full.
Slowly lower the bucket into the 4ft tank. let the fry swim out on their own accord
This will take nearly all day to do...
You should have minimal losses - I have done this many times and only lost 1-2 babies
Now, Add your OLD filter into the 4ft so the 2ft is now not in use for a little while. This will help with establishing the bacteria in the 4ft
In about 4 weeks - testing every 2-3 days & doing 10% water changes daily - you can remove the 2ft tanks filter and set the 2ft tank straight back up. Add your BN adults back in & platy adults if you want to.
When they lay the eggs, take dads cave & plop it straight back into the 4ft BEFORE the babies hatch. this will reduce the losses from moving susceptible fry from the 2ft once they have hatched
Dad will stay with the cave & mum will have the entire 2ft to herself for a little while
- I have done this for all of my spawns over the last 2 years and have had a 99% success rate & dont have to worry about catching itty bitty babies all the time