The oldest tank in my personal experience is my 10 gallon microreef. I set this tank up in 1991 to try out keeping a small reef aquarium. Orginally, the tank was set up with an ordinary HO powerfilter. Lighting was 2-15" NO bulbs. In 1994 a plenum was added along with a skilter. Lighting was upgraded to 2-28w PC bulbs. In 1998 the skilter was replaced with CPR backpack. In January, the backpack was replaced with a TF100. In addition, a small sump was added in January, along with a wavemaker and additional powerheads. This tank looks better every year that passes by. All the orginal live rock is still in the tank, nice and purple. The centerpiece of this aquarium is a pagoda cup coral that I lucked out on finding. Originally, in 1992, the coral was about 1/2" across with about 6 polyps and was attached to a huge piece of rock at the LFS. I broke it off and have had the piece ever since. It adds about 1/2" per year around and is now about 5 1/2". In addition to the pagoda cup, other "old" corals include a Pocillopora that started out as a frag and is now baseball sized, a colony of encrusting pumping anthelia, and a blastomussa. Although I have many other larger tanks, the 10 gallon is my favorite as it has done so well for so long.