Help me reduce nitrates

Tank has been cycled and running since October, started with dry sand and dry rock. I've struggled with high nitrates since the beginning and thinking this is what caused some of my invertebrates to die (2 emerald crabs, skunk cleaner). I was overfeeding at first but I have cut back the last 3 months. I started using filter socks to help clear up water and such, I change them every 3-5 days (mostly 3 days). Also run a Reef Octopus skimmer 24/7 (classic 150 int model) and empty it about once a week. Before I started the filter socks there was gunk settled in my sump so I completely drained it and scrubbed it top to bottom about 2 months ago. I tried growing chaeto in my sump and it just slowly died off, shrunk, and tore apart getting tiny green/white stringy pieces everywhere in my sump and DT. I threw that stuff away. I've been using Instant Ocean Natural Nitrate Reducer for about a month to see if that'd help over time like it's supposed to (no changes as of yet..).

At first I did 20% water changes every 2 weeks. Then I tried 20% water changes every week. No changes so I went back to every 2 weeks to cut back on so much salt consumption. I vacuum the sand bed to get detritus out the best I can every water change. This past week I've been getting desperate and did three 20% water changes (2 days apart from each other). It dropped it some but not much at all.

Tank otherwise seems okay. I have purple and red coralline algae spreading like crazy across live rock. Water is really clear. My first fish seems healthy and has grown a good bit. Went through a diatom bloom that passed pretty quickly. I do have a few patches of hair algae growing atm. I've also noticed some spirorbid worms that popped up in the last week.

Tank stats: 75g with 20g sump, 2 clownfish (had a lawnmower blenny for about a month and he died), 1 emerald crab, 1 peppermint shrimp, 5-8 snails.
Ammonia: 0
pH: 8.0 - 8.2
Salinity: 1.024 - 1.025
Temp: 78 F - 80 F
Nitrate: 40 - 80 :(

I really want to get more fish and invertebrates but haven't gotten anything new in a couple months trying my best to get my nitrates down so it's a more healthy stable environment. I was gonna do a 40% water change next week but I read this idea and couldn't quite understand if this really would help more.

Quote:

Example from my 55-gallon Reef : Make up 20 gallons of fresh saltwater in a trashcan in front of your tank. Drain 10 gallons of tank water into the 20 gallons of new water, and let that mix. Pump 10 gallons of that water back into your tank, and let the power heads mix that water up in your tank for a minute or so. Then repeat this three more times. Dispose of the now polluted 20 gallons of water. Make up another 20 gallons of fresh saltwater, and repeat this procedure. As long as your temperature and salinity match the tank, your inhabitants won’t be affected adversely, and with each rotation of water, the nitrates are being diluted and removed from your tank.

Source: http://ift.tt/2jjKMZp

I'm open to suggestion, comments, and/or pretty much anything at this point.

Thanks!