With the exception of the dwarfs, I am not sure you can oversize a tank.
As far as what else you will need to start the tank, off the top of my head I have included a list of my personal choices and
here is a link to a similar discussion:
- a water source for RO/DI water. ONLY use RO/DI water or distilled water, NO filtered tap water.
- containers for a
minimum of 20 gallons mixed saltwater and 20 gallons fresh RO/DI water
- more salt
- hydrometer
- test strips (for ammonia, nitrite, ph and nitrate) - you can use reagents but I have a strong preference for the stips.
- I would buy another 50 pounds of LR for both filtration and housing for an octopus.
- New sand (see my notes below). I prefer a thin (1"-1.5") sand bed that is regularly stirred there are other choices and you can use what is offered but I would rinse it well, in small quantities with saltwater and use less then the amount provided. IMO bottom substrate is just a nitrate storage unit and provides little benefit.
- a couple of 5 gallon containers to haul both fresh and saltwater for water changes
- a couple of 5 gallon buckets to use for removing saltwater
- a siphon for water removal (don't try to use one of the automotive ones, the tubing is too small, I like the ones with the squeeze bulb and wand pipe on one end)
- algae scraper metal if the tank is glass, one
made for acrylic if not and extra blades (you will see a lot of algae the first year, somewhat less after the tank settles)
- gloves are helpful when handling LR to protect your hands, I have them but the cuts on my hands would show I was lieing if I said I used them much.
If you are going to buy it in parts and assuming you buy the rest, you DON'T need (and likely should not use):
4' Wood housed Metal Halide Light with (2) 175 watt bulbs, cords, ballast and fan $200 (too much for an octo tank - just use the PC's)
(1) Red Sea Prizm hang on the tank skimmer $50 (optional - there is another but if you buy the whole kit this would be a backup)
(1) hang on the tank fluidized bed sand filter $35 (optional - not a necessity but some people find they help keep the nitrates down. Again, I prefer adding LR if more filtration is needed and keeping the set up simple.
(1) Rainbow Life guard UV sterilizer housing, needs new bulb and ballast. $40 (recommendations are to avoid UV filtering with an octo but you could use it on your water storage bucket to help fight bacteria and mildew).
(1) large ModuFilter Canister and 2 pleated filters and ceramic bio core $50 (semi-optional - if you have LR, a skimmer and a sump you don't need a canister but will need something like a simple filter sock and charcoal bags where the water dumps into the sump. You
will need a canister if you do not use a sump)
(1) S.O.S. Over flow skimmer to bring water to a sump 600 ~800 GPH $25 (you WILL need this if you feed the sump this way but it is far better to get the tank drilled and use gravity to feed the sump, using two pumps to exchange water is a very wet and precarious combination)
(1) 1400 GPH overflow, Acrylic, $30 - you will need this if you don't drill the tank but, again, it is better to set up a gravity fed system with holes in the tank - siphon loss is an on-going problem with external overflows. This was likely filled with the skimmer mentioned above to avoid the siphon problem but adds the issue with two pumps that cannot be finely tuned and stay finely tuned or one pump goes down while the other remains running.
Around 150 lbs. of Live Sand would be $2 a pound small quantities, larger quantities (Over 50 lbs ) $1 a pound - personal preference only, live sand is often desired but I prefer to start a tank with new sand and not with the build up in existing bottom substrates. You might want 5 pounds to put over new sand but I typically just put in new even when I am upgrading my own tanks.
Some AgroCrete hand made caves and rocks, cured and dry priced by the piece, from $5 to $15 (optional- personal preference again, I would rather see you use another 50 pounds of good live rock but
sometimes the DIY rocks are very nice and, if properly created and maintained can provide similar benefits, however, this says dry priced so I am assuming it has no current biological benefit.
5 Gallon Liquid Doser, for calcium and such. $15