main thing here is the current rating of individual item draw
and also the current rating the extension wire/plug/socket/mcb
can stand without overheat the plastic.
so simple calculation :
1) Return pumps x 2 (50W x 2 =100w, 0.5A)
2) Chiller pump (50w, 0.25A)
3) Skimmer pump (100w, 0.5A)
4) Wavemaker x1 or x2 (25 x 4 = 100w, 0.5A)
5) Main Lightings x2 (150x2 = 300W, 1.5A)
6) Cabinet lightings x1 (20w, 0.125A)
7) Chiller (300w, 1.5A)
8) Calcium reactor (in future) (50W pump , 0.25A)
9) Controllers/monitos of any sort (0.2A)
10) Fan (0.2A)
total approx 6A.
Mcb rating is 20A/15A
Socket/Extension rating ??
wire rating??
so the only concern here will be the extension/socket/wire/plug that you
use. As it may not withstand the 6A rating.