got my 25 trinadad olives [MOD EDIT: USE SCIENTIFIC NAMES PLEASE. Trinidad Olive = Holothele incei ] in a communial setup,
just for curiosity i wanted to no what size enclosure is needed for them,
they are 2,5cm and the tank is 20cm x 30cm.

I would give a bigger enclosure for that many Ts, they will grow too

I will go for something MUCH bigger. Something like:
915mm length
325mm width
375mm height

keags wrote: got my 25 trinadad olives in a communial setup,
just for curiosity i wanted to no what size enclosure is needed for them,
they are 2,5cm and the tank is 20cm x 30cm.

You can keep those guys together until they reach 3 to 4cm, then they need to be moved, I would say double the size of your tank that it is now, 60cm X 30cm X 220.

Give them lots of places for them to hide.

@Keags sorry for using your thread

I am lloking at starting a communial with Trinidad Olives as well. The T's sizes is in the range of 2.5 cm. How big an enclusore wmust I supply for the 7 little friends.

i would say about 50cm L x 40cm W and 40cm H.
other people woul differ.
if the tank is to big you H.incei will not all find food as the food skatters.
some will eat and some will not.
thats my oppinion.