Re:Tank Lighting cycle, but NITRITES

I really enjoy reading all of your helpful information!! I actually aquired the desire for seahorses after reading and poking around the Ocean Rider site!!

I have one more question!!

After the morning feeding on Friday, I siphoned out the leftovers and did a check on my levels. My Ammonia was 0ppm, but my Nitrites were slightly elevated. I immediated performed a 20% water change and took my siphon and went over the sand bed just to be sure. That night, I performed another nitrite test and the reading was the same, before feeding. I performed a 10% water change immediately and fed only at the feeding station so that I could be sure I got ALL of the left overs.
I tested again and the level was up to .50ppm!! I changed the water (10%) once again and tested, the level was down to .25ppm, I still wasn’t happy so I went to the Amquel +. At 3am this morning, the level reduced to 0ppm, and I went to bed. When I got up at 8am, I tested and the level was at .25ppm, AGAIN!!!!!
I am getting really frustrated so I did a 25% water change, and the nitrites were still at .25ppm!!! The seahorses are not gilling heavily, and they are swimming happily – could my test be goofed? It is a pet store "chemistry set" with vial and matching the cards – I believe the brand is "API"?

THis is driving me nuts and I am constantly checking the water with no change, even with amquel and the massive water changes. All other levels are at 0ppm, the hydrometer reads at 76 degrees and 1.021.
The tank has been cycled with live rock and live sand for over 4 months, I have 6 hermit crabs, a conch, and 3 peppermint shrimp, and a target mandrian that is about 1" long. The only food that goes in to the tank is the mysis, left overs are siphoned out, as well as the poop from the seahorses.

I have a mechanical carbon filter, a sponge filter, a protien skimmer, and a heater for equipment. I change the carbon filter once a week and clean the sponge with mild water 2x a week.
What could be the problem?? PLEASE HELP 🙁