Backyard Aquaponics
A place to discuss aquaponics

Most small pond pumps typically have connections for either 13mm or 19mm flexi pond pipe. I would try for 19mm personally but you will probably get stuck at 13mm or even smaller with 400LPH.
I doubt any small pump you brought on that sort of size would support or need 25mm pipe.

15-20mm PVC would be fine for the GB feed given you will probably have a 'T' joint.
Returns wont matter much unless your pump is oversized - 25mm down from siphons should be fine.
If you are worried about flooding you balcony go a bit larger and go without the siphons..

On the siphons note that they will not discharge all your grow bed water (will be a bit left behind) and your water will not come to the top of the media/container if you set up correctly so your GB water volume will be less again from the 40L (and max worse case 80L) that Mel suggests - which is all good for you.

the configuration looks fine. Try to keep your pipes and discharges over the Fish Tank where possible just in case of leaks.

[edit] I would use PVC as the fittings go on outside of pipe. With small poly pipe fittings the connectors go inside the pipe and these create blockage points (and it is very easy to block 19mm poly pipe, and even worse trying to keep 13mm clear. 15mm PVC would most likely be your best bet but not sure how common that is in China - pretty common and inexpensive here in Australia.