Nitrates

So, I went away about 3 weeks ago and my tank was neglected, I had someone feeding my fish. When I returned, My nitrates were 100!:doh: I have been doing daily 5-10 gallon water changes for the past week. I have a 75g tank and feed about a cube of frozen food each day since I have a heavy bioload but I make sure the all of the uneating food is removed from the tank. I got the nitrates down to 25 a couple days ago. But for the past 4 days, I haven't been able to get the nitrates to drop even though I changed 30g of water. I also added a reef octopus bio pellet reactor, which is paired with my reef octopus 150 protein skimmer. I siphoned the sand bed both yesterday and this morning yet my nitrates haven't dropped! I did test my ro/di water and freshly mixed salt water and they both have zero nitrates. Also, I change my filter sock daily. Does anyone have any Ideas

Oh, and I forgot to add I am using Salfrite test kits.

-Cooper