Tanisheily, the first thing you need to do is verify your readings with another test kit. Have your LFS check the water.
Providing your readings are accurate I would say:
1. Cannisters are not helping, the rings are going to convert any nutrients to more nitrates. We don't need more CONVERSION here with your light bio load, what we need is more nutrient EXPORT. I would SLOWLY remove this media in both of them over the period of 90 days, a little at a time. Leave the cannisters running empty for more circulation.
2. At the SAME time, add another skimmer. Also make sure your skimmer is working; should be removing dark sludge every day for you, if not then adjust it or find one that does.
Get one bigger than the gallon rating on the box. Choose wisely, lots of overated junk out there.
3. Tell me about your substrate; 2-3 inch crushed coral beds, dead sand beds can all be nutrient sinks/adding nitrates faster than water changes can keep up. Hobby is supposed to be relaxing, after all. A properly balanced basic marine system should only require monthly water changes or less.
4. You may need to look into a deep sand bed (DSB) located inside a refugium (not in the display) there is much info available on the web for that, use your search engine. Also look into an algae filter, which can also be located in a refugium/sump below the tank. Try a big wad of chaetomorpha (not caulerpa) in a reverse lighted sump, that will serve as a great export filter for your pesky nitrates. Plants love consuming fertilizer! Be advised, you don't need zero readings for a basic FOWLR system, but agree 100 is too high.
5. You are gambling with adding uncured seawater; intro of pesky pathogens a real possibility here. Cure it for 4 weeks in sealed fish-safe containers in total darkness, then aerate for 24 hours before using.
Feel your frustration, but this can be conotrolled/balanced.
Your questions will lead to success. More info below from my files:
High nitrates are more of an indicator of total dissolved organic compounds and overall system health, such as PH issues. Some species are indeed less/more sensitive than others to DOC's and nutrient/metabolite buildup. For reef/invert systems, most target less than 5 ppm and they often keep sparse fish populations in those tanks to help limit nutrient input. For more robust fish systems nitrates are less of a concern; a 20 to 40 ppm reading is common and can usually be maintained/lowered with efficient skimming and scheduled water changes. While many fish are likely not harmed by moderate or even higher nitrate levels, IMO higher nitrates almost always go hand in hand with a low PH; and PH/alkaline reserve is a much more serious parameter that requires careful attention in all marine systems.