No, living starfish can not cause nitrates.
If you have crushed coral, gravel vac it really well, half one day , half the next week. Then, replace it with dry sand.
If you have sand in the tank already, very lightly gravel vac it with really low flow and be careful. You will get some very dark brown/black stuff out. Starting fresh is good, and will balance your aquarium out again.
Water changes do not clean out the goop that gets clogged in your sand bed, in your filters and other crevices. If you have filters, change out half of them. If you have a canister filter, do your regular monthly maintenance on it. Change your carbon out once a month. (every 30 days)
If you don't have macroalgae, get some macroalgae for the display tank and add a decent light fixture. If you have a sump, add macroalgae to the sump portion that doesn't house your return pump. Add a clip on light from wal-mart and harvest the macroalgae often. The higher the intensity the light bulb in the clip, the better - the macro will grow faster, especially with as much nitrate as you say you do.
Another option is to buy a better protein skimmer. Obviously the one you are using right now isn't working very well - so change it or get it working properly again. A good skimmer should pull out a whole cup of skimmate once every three days.
Another option is to research and use an algae scrubber. A properly built algae scrubber will reduce or eliminate nitrate and phosphate through the natural means of growing hair algae on a screen. (please consider this option, and consider building it inside a 5.5 to a 10g tank instead of free standing.) You'll understand these directions if you research algae scrubbers.
Another thing that someone in a few minutes is going to mention is to double and tripple check your test kits. Test kits are more often wrong than right when it comes to testing for nitrate and phosphate. If your test kits are old or stored improperly, you could be looking at inaccurate readings. Double check it with your live fish store and see if there are any discrepencies. If they are wrong, consider getting Seachem test kits, Red Sea test kits or better yet - Salifert test kits. The quality of the test kit depends on how much you pay for it. If you want accurate readings, you will pay...
Instead of saying "high" and "low" Post your actual results and readings.
What livestock do you have? How long has the tank actually been set up? What substrate are you using? What kind of filtration are you using? Etc. Etc. Etc. .
