Nitrate will not go down...

[whatcaneyedo]

A skimmer doesn't have to cost a lot to work well. Mine was used and I think I paid about 40% what it would have retailed for. Several of your fish are going to outgrow your current setup and their current tank mates but I suspect you might already know that as its already happening. I've never owned a porcupine puffer but the snowflake will eventually be able to eat many of your other fish so I hope you have a plan. You've got just as many fish in your current system as I have in my 240gal system so I'm not at all surprised that you're having nitrate and phosphate issues. My system does quite well with a skimmer rated for 400gal with a similar fish list so you might want to consider that when you decide to upgrade.