1st, I dont know how experienced you are but what i will say will be like you dont know much about exuipment of saltwater aquariums whatsoever, so i apologize if this is annoying stuff you already know.
Well, 75 I think is a very good size, allowing more time for mistakes to have a effect, which are inevitable. The only dimension difference for 55-75 though is maybe and inch shorter, and 5 inches skinnier. Unless youre concerned about how far is protrudes off the wall, ur much better going with the 20 gallons more; for the extra water, and the surface area is greatly increased. But it all depends on how many fish you want, if you want corals, and all of that. You can have roughly a fish per 10 gallons, depending if its a neon dottyback or a tang, it will obviously differ, where as a tank is not at all comfortable in 10 gallons {and pollutes it much faster than most people want to deal with} and you could have around 2 of the other little fish. But do your research, and no matter what the size, you can be successful, and wont end up with a bunch of dead things. Saltwater is a VERY rewarding, beautiful hobby and well worth all the money and work that they require. For my 75, the equipment I have is:
RedSea canister filter- Two long tubes come out the back of the cabinet where the filter resides, and into the tank, intake on 1 side, discharge on the other. First box includes two plastic nets, foam, bag of carbon. Second includes 4 sponges. Very efficient.
Prizm protein skimmer- I preach about these things. Once you have seen what they remove in just one day, you would never want to imagine how your animals would feel if all that gunk was actually in your water!}
2 powerheads- Vital for helping along your skimmer to get out all the waste corals and such produce, plus species such a gorgs. need good flow.
Good heater
Coralife lighting fluorescent and actinic, 3.46 watts/gal. sufficient for most corals im interested in, but there are MANY choices. Also hooking up a timer may be worth your while.
80 lbs of sand, fiji, aragonite for filtration and making it homey
Lots of LR= home to the fishes and lots of neat/wierd things you wouldnt see unless you were under the ocean.
And of course a nice big net, a saltwater test kit {i test for calcium, alkalinity, ph, nitrite, and nitrate weekly, and occasionally for copper and phosphates. But if there is no problem with the last 2 in your tap, then it wont be necessary to but the kits, just have the LFS do it once.} and any supplements you will need, such as calcium and trace elements, essential for corals! An eyedropper with measurements has been a big blessing for me in feeding my corals and measuring stuff. Research food for your animals and make sure its very nutritious whatever it is to enhance color. I like brine shrimp plus frozen by Formula Foods, and of course the live brine!
And of course a sump and/or refugium always makes life easier but im doing it the old fashioned/non-lazy way and everythings great. And then of course theres all those fancy contraptions like home made sea salt mixers, calcium and alkalinity reactors, and so forth, but if u r worried about the cost, its probably isnt on your 2do list.
The main equipment and LR and LS ran about $1050 at my LFS. Sticker shock! Plus the money you wont be able to resist spending on all the beautiful specimins u will c @ the LFS, its about $100 a week for me!
Hope I was of some help for your basic equipment knowledge of a reef tank!
