You don't see them like this for fresh water too often.
This system is in my basement - below a 110 with a Parrot Cichlid and 2 Perruno cats. The reservoir here is a 135-gallon.

Just set this up (May, 2010) out of stuff I had laying around at home and in my mom's garage. I still have a lot of tweaking to do, but I'm pretty geeked about getting back into the hobby.

The idea here is to increase the volume of water in the system. The system total is near 250 gallons now, just to buy me time between water changes. Also, the trickle box used to sit in a 10 gallon tank, which needs to have water added every day in the winter time to keep the pump from going dry. This system can lose 100 gallons before that happens.
This is nice because I presently haul water to my house in 32-gallon trash cans.

I haven't put too much thought into the cubes above. They will define themselves over time. The cubes could be for fish even too small for the grow out tank. I plan to put dolomite or limestone in one to buffer pH a little.

You see the one on the end is full of Aquaclear sponges. If I need a quarantine tank or hospital tank, I can pull 30 gallons and a seasoned sponge and have a cycled tank set up immediately. Or give sponges to friends when they start up tanks. I can use the cubes for activated carbon or whatever I can think of.

I can do water changes from my basement instead of my living room now. I also plan to hook this into my plumbing so that I can do massive water changes in moments.