To begin, I have not seen a tank with those dimensions, but I may not have been paying attention
.I also don't think it is wise to use a six inch sandbed to obviate lighting problems. I tried a six inch bed once, and did not like it. Even the 18" depth, and narrow 12" width is very difficult to decorate. I myself have a standard 50 gallon tank, which is 36"x18"x18". This tank is easily available for just under $100.00 here in town. Using a four inch sandbed I am left with a 14" water column, which I light with a 2x96W PC fixture. That is my two cents worth anyway.
