Wow, I really wish you hadn't just poured that sidewalk.....If you could take out a two, three foot section, then bridge over with wood then you could open the area on the otherside of the walk way also, how great would that entry be, to walk over the fishies! Some canna in close to the garage...... I agree, the bigger the easier, I have two small ( 1-2")baby koi in a 20 gal in my sons room, can hardly keep it balanced, gonna send them out to the pond ang get small trops in there. I didn't notice where you were located, would tropicals be an option, or matbe just some goldfish. It can be done in that space, just not ideal for koi. You have to account for slope loss of volume, and no rock, that will take up volume too. 3X12x...24"(house)-36"(Drive) I would try, but no koi. I would shoot the water from the house towards the drive, with a BD there to skimmer/pump at the drive end, then back up to the house end, not a water fall, a PVC with holes going verticle and horizontal to flush the water downstream, that way you will get somewhat of a river effect and the fish can get some exercise. My ponds have always been too small and shallow for ideal situations, I'm just a hobbiest, enjoying my fish, please understand though......it's a daily chore, not weekly....daily.