well this might sound harsh, but you should pick up a book or two from a library and thumb through them, or ask some questions on this forum before you actually start making them, just to make sure you have a real 'need' for the filter(s) in question. i started my fish keeping hobby in 92, and by 94 i had a salt tank, and had to read book after book to get answers i needed.
a fluidized bed filter is an all-one biological filter, it can replace any bio-filter you already have, or supplement anything you already have. i have used a bed filter on a couple of tanks, [and did try the supplemental ideal, and the replacement] and have been very happy with its performance in both cases. its kept everything right where it should be (amm, nitrites and nirates). during the nitro- cycle, ammonia is generated, then bacteria turns that into nitrates, then other bacteria turn that into nitrites. nitrites aren't very deadly to fish in small does, and nitrites are removed through regular water changes, or through absorbtion by plant-matter. some people hate doing water changes and thats how/why the de-nitrator came into existence, it claims it can remove most of the nitrites in a tank and leave you with less aquarium keeping duties, or so they claim. if your doing a saltwater tank, and this is a esp. true with reef tanks, water changes are a must regardless if you have a de-nitrator or not. if you running a high shelf reef tank [uber expensive] and then having a denitrator is a security device, and that i can understand and agree with. but just to have one, to have one makes little sense to me when you have a bed filter. however for the record i think building a bed filter is completely un-nccessary, given how temperamental they can be regardless [with a commercially made and tested unit] and the price of current commercial made units and the new features they have, like: self regulation to achieve optimal fluidization, which on series -1 bed filters had to be done through trial and error and if you go
diy with it, you will run into the same problems and/or all new ones.
alot of companies are building 'reactors' these days; co2, phosban/chemical, kalkwater, the list goes on and on, and if you running a tank for your local city-aquarium then buy every reactor on the market or build. but if yore not then it makes little sense to bother with every single reactor. the heart of a chemical reactor is that it forces water through granulated carbon, something a 10dollar
HOB filter does. maybe that
hob filter doesn't have that great of Efficiently but it does get the job done. some things need a reactor (like co2) and maybe kalkwater but in my humble opinion most other processes don't need them. esp in the carbon and phosban department. i have heard this myself from the forum people, and from a magazine a while back, but carbon only stays activated for no more than 7 days, after day 7 it has lost most (if not all) of its ability to absorb chems from the water anyway.
would it be safe to assume that the bed filter, de-nit, and chem reactor are going on your 75gall tank? what do you have on there now ? and have you ever heard of garf.org or the berlin system of saltwater tanks ?
But DIY is the way to go with it... 
