I've thought about doing the same thing using one of those 8 to 9 foot diameter plastic stock tanks they sell at Tractor Supply. I think they are about 18 to 24 inches tall and would make a cool saltwater pond. If you are talking about making it a reef tank, then yeah, it will be highly expensived due to the lighting, skimming, and pumps you would need. For a FOWLR or softie tank you could get away with cheaper lighting, possibly using a couple of those solar tubes. I've also thought of building a concrete slab off of the back porch. That way it would get natural sunlight and I could build a retractable awning of some sort to keep the tank covered when needed. Only issue then is the south Texas heat so I also thought about digging a pit for the tank to sit in the ground to help keep the temperature more stable. I think it would be neat, maybe have a deck around it with the equipment underneath out of sight.