Check your water parameters. You know "right where they need to be" doesn't help. My guess is your bio filter crashed & you will have nitrite spiking & nitrates through the roof. You gotta keep a sharp eye on young tanks, especially at the end of the cycle. It can happen overnight. Especially when using large fish to cycle.
What filter are you running on this tank? Where is the filter intake (corner, center)? The 20gLongs are hard to filter properly & keep stable because of the shape (long & shallow). I run an AC50 & an AC30 on mine, but I still have to run a small powerhead in the opposite corner to keep the water moving.