for the livestock you have, unless it's purely an aesthetic thing, I wouldn't add more if it were me. There are some small strip LEDs that you can supplement color with (check the ecoexotic stunner strips). }
As far as nitrate rollercoasters go, that's a little weird. IME, unless something dies and is rotting, it tends to creep and build, not vary wildly. If it's real, and not test error, it may have to do with the small tank volume and high frequency of water changes that you're doing. What kind of swing are we talking about?