Prior to knowing about ARCSA and the training classes they teach, I installed this 1,000 gallon tank with a rainwater basket under the top cover.  I used 2.5 inch pipe and ran it from the gutter to the underground drain below.  In between, I cut in a ball valve so I could manually leave it open during the first rain to flush off the bad stuff and then close it to divert clean water into the tank.  I ran the overflow back down and under the ball valve.  This was before I knew about downspout filters and first flush devices.  Bottom line the system works well and the water is clean.  It easily passes through my compression pump and I use the water to augment my regular irrigation schedule.  The tank was originally black and we painted it to blend in with the home color.  A good example of DIY with a few trips to the local irrigation store.