Buy or build a sump.

if you choose to go the rubbermaid route. instead of baffles you can put a smaller containers in there for things you need a set water height and let it overflow into the main rubbermaid.

 

After I asked I thought of that but thanks sounds like that might be the way to go.

 

+1 to using the 125 for your sump/fuge. How much do you plan on getting for the used 125? Will that out weigh the cost of the rubbermaid option? And do you have someone lined up to buy it or are you planning on craigslisting it? When i was looking for a used tank, I found that there were a lot of tanks, at least in my area, and most people wanted more than i was willing to pay for a used tank when i could buy a new one for not much more. just my 2 cents

 

The last few sumps I've used have started out as an acrylic aquarium buy off of craigs list.