Kyle,
Along the line from what Les said. You very likely have something in your tank that has accumulated a lot of organics that is creating the nitrate problem. The high organic load is too much for the denitrifying bacteria in the live rock to handle. The garbage can full of oolitic sand may be part of the problem.
If this was my tank I would do the following things.
1) Crank up the skimmer so it is skimming very wet with a lot of skimmate. Like a gallon or more a day.
2) Install a particle filter to trap all that goes through the overflows. I use a filter floss made from quilt batting material from the sewing supply store. A lot of other people use filter socks. Either will work, just catch everything you can.
3) Get a powerhead like a maxi jet 1200 and use it to thoroughly blow all the sediment you can out of the hole, crack and crevasses in the rocks. This will be caught in the filter floss/sock. Do this every other day or so for a couple of weeks to get as much out as possible.
4) Take the denitrifying sand bed out of the system.
5) Add a fresh load of activated carbon and change out often. The frequency depends on how much you are using and if it is a bag hanging in the sump or in a flow through chamber.
6) Continue with the frequent water changes.
7) Feed the fish every other day or less, use dry food or rinsed and patted dry frozen food.
To bump up the magnesium, get 1 or 2 cartons of Epson Salts. It is magnesium sulfate and will get you back up to where you should be.
9) Have your water checked by someone else with known good test kits to validate if your test kits are accurate.
This should get your tank back on track.
Rick
