20 gallon tank help

Well I am new to TRT and I have a 20 gallon long saltwater hopefully soon to be reef tank I set it all up awhile back and I was super busy with work and school so I didn't keep up on maintenance but I want to get it all going again my problem is that my nitrates are off the scale I have been doing water changes weekly an clean bing my tank and HOB filter weekly. But I heard somewhere that If you have a hang on back filter and live rock and live sand that it could cause extra high nitrate levels since it's 2 types of filtration I just always thought the more the filtration the better. I have also been adding instant ocean natural nitrate reducer.



My calcium is also high but I haven't tested the alk yet I'm waiting for the test in the mail



Things I have

5-10 lbs of live rock I plan on getting more

Live sand

Aqueon quit flow filter 30 gal or 50 gal I forget

T5 lighting



I also ordered a protien skimmer a powerhead a carbonate hardness test kit and a phosphate test kit that are on their way



My questions are

What can I do to help get rid of my nitrates?

Is it better to get rid of my filter and just use live sand live rock and my protien skimmer?

Where can I get a magnesium test kit?

I will have more questions I'm sure when I get my alk test kit



Thanks in advance -Josh