I have a 20 gallon tall reef tank. I started it out coincidentally using an aquaclear 110, and later built myself a sump(8 gallons).
The aquaclear will do fine in a either tall or long 20 gal
I used chemipure elite, purigen(seachem) and some floss media , and my water used to be sparkling. I dont think any filter makes the water as clear as the aquaclear... However, as saltwater buds know, there is a need for a sump, sooner or later, so I built myself one inside my cabinet. I stopped using the aquaclear because I got a clamp on 150w metal halide(clamps on the tank's back glass) so I could not use it with filter because they bouth use the same glass as support. I have around 20 pieces of corals, amongst sps
, anemones. I can say that duoc 9119 said it right in regards to the light. If you want the perfect light for the high tank, the metal halide is the choice. It might be a couple of buck more, but on the long run, you will spend less on your corals, which will not die because of light insufficiency...If you are going to start a reef, start the right way...The tall tanks, especially reefs are the nicest ones in my opinion....good luck!