The most helpful thing that I have learned in this hobby... is do everything SLOWLY!!! I understand the desire to correct any problem as soon as possible, but in most cases changing anything quickly causes even more problems.
Don't rush here, if at all possible! Yes, there are always exceptions... but for the most part when you do something, sit back and watch for a while to see how what you just did ends up changing things (good and/or bad), befor going on to the next step.
Right now, resist the temptation to add anything new to your tank. Let your system mature, while you watch what you are working on now. Do another water change (another 15 gallon would be fine) about 2 days after your last one. Test. Watch for a couple more days... then take things from there.
It has been the HARDEST thing for me, not wanting an INSTANT stocked tank... looking beautiful right away. Been there, warn out the T-Shirt. Lost too much, and learned the hard way. Things that move slowly in this hobby tend to be more stable, and that makes our animals (fish, Anemonies, Inverts, Corals) much happier and healthier... and they then reward us with their beauty.
Okay... WAY TOO WORDY, sorry. Someone needs to SLAP me!!!