hi all, im setting up a 72 bow and looking at the different ro units and sizes i will be running the 72 and a 55 gal off the ro water, what size do you all think will be ok for me, thanks in advance for your help....will i need a bladder holding tank also?

You will need a storage tank because when you want to do a water change you will need to have the water already produced and mixed with salt. If you plan to just use it for an automatic top off and never want to do a water change, you won't need the tank. I store my water in a large rubbermaid trash can that's always been only used for this purpose of clean water storage. I suggest getting a unit that does DI as well. The deionizing chamber will strip the water down to 0 total dissolved solids. 75 GPD should be fine for you. You can always change the membrane if you want to step it up to 150.
Bulk Reef Supply - Bulk Reef Supply
Cultivated Coral

hey Jarred thanks for the site, great prices....

Sure thing Jeff. I have bought from both companies and they are great to do business with. Cultivated Coral is a sponsor here and Bulk Reef Supply have a great selection of RO units and related filter media. That's where I get a lot of my stuff. Cheers

By the way, Jeff - the dual TDS meter is a great investment, takes the guesswork out of when its time to replace the DI media and filter cartridges in the RO unit.
For example, for the past couple months my setup runs about 160 PPM into and 0 PPM out of the RO/DI unit. I checked it yesterday and its running 14 PPM out. So I know the DI is getting exhausted. My target for product water is 0 total dissolved solids out of the units. I check the TDS at least once or twice per week to catch it early. This keeps algae from blooming and coral health & growth maximized.
I also run a pressure gauge coming out of the unit, though thats not necessary. And I also run an auto shutoff valve to relieve pressure on the membrane when there's no demand. This gives it a better lifespan. And of course, checkvalves in a couple places. I have a flush kit as well but dont use it, though I probably should.