For plants and macroalgae, the essential macronutrients include nitrogen, phosphorus, and potassium. Since I had undetectable amounts of nitrogen (no ammonia or nitrate) and yet phosphorus remained in the form of phosphates, I realized I had an imbalance, and the limiting nutrient was nitrogen, which was too low to allow the chaeto to utilize the phosphates. If I wanted to spend the time, I'm sure I could have found published ratios for nutrient uptake by macroalgae, and then do a mass balance on N and P, but I didn't. Instead, I just added a solution of KNO3 until I saw about 5ppm nitrates, and then just waited. Nitrates, in my opinion, are easy to get rid of, as they are removed by either macroalgae or by bacterial denitrification, so I'd rather add a little too much and then let it come down over time, making sure that all of the phosphates are removed.
I didn't do a cost/benefit analysis on the system, but it would be simple to do so. I used a 26W CFL bulb for 10 hours a day, a 50 watt heater (assume 12 hours on), and a 3.5W powerhead (24 hour). This equates to ~0.95kWh per day, and it took 5 days, for a total of 4.75kWh. At ~10 cents per kWh, it cost me $0.475 worth of electricity. I also used about 50 cents worth of carbon, 10 cents worth of stump remover and 15 cents worth of trace elements (5mL). (I also used ~$1 worth of water tests, but that is a one time cost). So for 20 gallons of filtered salt water I spent a grand total of: $1.23.
Assuming ~30 cents per gallon of mixed Reef Crystals, 20 gallons costs $6.00. So I saved a total of $4.73 by filtering used reef water. The only concern I have with doing this is getting the trace elements and ionic balance correct. You may have to test for potassium (which was added with the stump remover) as well as other important elements. I wouldn't use this water for a SPS reef without performing several tests, but I would be fine with it for a frag tank or soft coral reef.
