wrote in message . ..
I am looking to build a sump filter for my tank.
It is a 125 gallon reef.
I am curently using fluidized bed filters,
but want to upgrade to a sump with bio balls.

Don't you have a live rock in this reef now?
Why would you need bio balls ? This is strange...
Do you know how bioballs actually work ?

The problem I have is that I can't find a pre-made sump
that will fit into the opening of my cabinet.
I have a 10" opening that I can fit a sump into.

And this is square opening ? 10x10" ?

What are the dimensions I would need to have a sufficent
sump for filtration?

You do not build sump for a reef tank to have filtration there.
You build sump to hide a heater, skimmer and other stuff there.
Also, to increase the total water volume of your system...

Sump in a reef tank is NOT to be a wet/dry filter known
from the freshwater systems. Bioballs and similar media
are NOT used comonly in reef tanks. Live rock is acting
as a natural, biological filter...

Also, is there a type of plexiglass that I need to avoid?

No.