any suggestions on how to keep the nitrates down.
The filtration you have now is all very good at converting ammonia to nitrite and nitrite to nitrate but it's generally accepted to be pretty much ineffective at converting nitrate to nitrogen. Reefers mostly use live rock and live sand to do that last step in the conversion process but with a 45 gallon tank there's some question whether you could feasibly keep enough LR & LS to do the job.
An alternative would be to use a so-called Algal Turf Scrubber (ATS) of the sort developed by Walter Adey and described in his book Dynamic Aquaria
. I think that there is at least one company selling these systems but I can't recall the name. In any case an ATS is a simple enough piece of equipment that it should be pretty easy to DIY one for yourself.
Essentially an ATS uses turf type algae (think Hair algae with a crew-cut) to take up any and all forms of undesirable nitrogen compounds as part of the photosynthesis process. Additionally, an ATS will also take up phosphates and even some toxic metals as well as help maintain a high level of oxygenation and a proper pH.
Mechanically the system is quite simple but very specific in its requirements. The part of the system that actually does the work is just a piece of fiberglass window screening on which the aforementioned turf algae has been seeded (more on this below). The algal screen is mounted in a dump-bucket so as to maximize the agitation to the plants and illuminated in reverse phase to the display tank. For your situation a screen of 64 square inches (8" x 8") illuminated by 32watts of appropriate light (my guess is that a 6000K PC would work) ought to be more than sufficient. Of course there's no real downside to going a bit larger but try to maintain a light density of about 800W per square meter. Basic maintenance consists of scraping the excess algae off of the screen once per week. This weekly scraping (done with a couple of swipes with an ice scraper) accomplishes two things. First, it removes all of those nitrogen and phosphorus compounds that the plants have taken up. Second, this sort of algae grows most vigorously after being trimmed back so harvesting makes the system work more efficiently.
Seeding the screen is either the easiest or most difficult part of starting an ATS. The process is simply putting your screen in contact with an existing growth of turf algae for a few days - typically this is just another aquarium or even your own aquarium so the process is really simple. OTOH: If you don't have access to such a tank you may have a problem.
The most complete reference on this system is, of course, Adey's book. If you have access to a large library you may find it there. If not, the book is sometimes available on Ebay and can always be found on Amazon or ABEbooks.
You can also find quite a bit of information with Google searches on "Algal Turf Scrubber" or ATS but beware; there are quite a few nay-sayers out there but it has been my observation that they fall into two groups. First, there are those who have never used the system and are distrustful of non-standard ideas. Second, there are those who think
they have used the system but actually failed to implement each aspect of it. [The most common omissions have to do with using the wrong lighting schedule or not harvesting the growth regularly.]